You spend them on Recruit > Exchange.

Alright, below is a long list of what I think is useful without the order of which is the best. Decide them based on what you need and prioritize. Please don't take it as a gospel, think of it as advice. I wasted time for random forum post but here you go.

Note: With too many choice, it can be hard to decide. My advice is to reduce the option as many as possible based on your needs, then choose what you think is best. Also, no need to rush to get specific ninja, as many ninja listed here can be obtained passively from pack like 3v3, DB, Infinite ILlusion, and arena pack. Even if you can't get them, all ninja here aren't very important. They aren't position 1 ninja that will determine your gameplay, but they are mostly support. Furthermore, ninja points are time-based resource, so even if you have small amount, you will still get them later. You can even stockpile ninja points for future potential.

In general, if any ninja here require BT and they are obtainable from treasure, i recommend to get them from treasure first, to reduce the amount of ninja points to spend.

- Yagura: Can be useful for nature training. There is a ninja in the opponent team who can grant them immunity to death. The idea is, you use Yagura to clone that specific unit. This will inherit all their skills (except mystery), which include the passive to also grant your team immunity to death.

Buy or Not?: Maybe not, because he is expensive (1300 point per frag), and you can get him from Jinchuriki Treasure 1. He is a purple unit there, meaning you can get him with 40-60 seal scroll (more if you are unlucky to get other purple unit).

- Hidan (BT or not): Can be useful for beginner in all three Infinite Illusion. Even better if you have BT.

Buy or Not? : Sure, if you are not going to get other useful ninja for Infinite Illusion in moderate amount of time, he can be the one. If you are only planning to use him during early game, don't buy him. This is because, he is available via test ninja, although test ninja version will be much weaker, but it's still a good choice for beginner. Another reason is because some people (maybe you) obtained him from the 20 draw of the seal treasure.

- Asuma Wind Blade (BT or not): A classic choice for blitz team, to gain chakra and some atk/nin buff.

Buy or Not? : Yes, but no need to rush. The point of blitz team is to make your fight faster, typically for PVE event like TI, SA, wanted mission, RJ, etc. It is not mandatory to have a faster fight. Furthermore, you can obtain him from passive source like 3v3 pack and monthly arena six path ranking.

- Ino GNW (BT): Can be useful to support Obito rage. She provide control from mystery, healing and debuff removal (important for obito team), gives atk/nin buff to secret jutsu user (including obito), and provide extra chakra from her chasing.

Buy or Not?: Sure, if you are planning to use Obito rage, you can utilize her. She c*so be support for Madara ETR, which is also a secret jutsu user. Additional note, she is very cheap with 200 point per frag.

- Choji GNW (BT): Nice position 1 for early game.

Buy or Not?: You don't buy him to recruit but rather to complete his BT. This is because new player gets 180 frag of Choji GNW from 7 days login. My advice is to BT him with vial until you get enough skill, then buy his frag to complete it. He is very cheap with 200 point per frag, so don't hesitate.

- Shikamaru Chunin (BT): Useful to support secret jutsu user like Madara ETR.

Buy or Not?: I wouldn't. Madara ETR doesn't really need extra def/res. He c*so reduce mystery cooldown, but is also not useful for Madara. He isn't useful for another secret jutsu user like obito because he doesn't need tankiness (to go into rage mode faster).

- Mifune (BT): A support for sword ninja. He is useful for shield/buff removal, atk/nin buff, and cooldown addition

Buy or Not?: Maybe, if you really need a sword support and don't have other choice. The problem is that he is expensive with 1300 point per frag. Will only buy him if you desperately need a support and you obtained 80 frag him from five kage treasure.

- Kurotsuchi (BT): Support for earth ninja, can be used to suppress immune or remove shield/buff, increase nin of earth units, increase enemy cooldown, and give extra standard randomly.

Buy or Not? : Nope. Her mystery can only target 2 opponent and has long CD. I imagine using her as position 2 and only using her mystery once in round 1, because she will not likely to survive until round 4. So, mystery is not that good. Her standard is useful but thrown randomly, I personally hate random buff. It's not a reason to invest in her with her two other perks. Furthermore, you may need to obtain her in treasure to reduce amount of ninja points to spend.

- Cee (BT): Support for hidden cloud or lightning ninjas.

Buy or Not?: Maybe, if you really need a support for hidden cloud. As far as im aware, not many support available for hidden cloud, so he can be a solid option. However, his frag can be obtained passively from 3v3 pack and arena. Furthermore, there may be better option, depending on what ninja you are supporting. Maybe a generic support can be better?

- Fu Yamanaka (BT): Support for anbu ninja to let them leech by ninjutsu attack, and provide little atk/nin buff to them.

Buy or Not?: Nope. If you specifically need leech, there are main character leech. Fire main let your units leech based on ninjutsu attack, while lightning main is based on taijutsu attack. Furthermore, he sounds like a specific support for anbu ninja, which not many exists. Shisui Susanoo BT is an anbu position 1, but doesn't make sense to use Fu if you have expensive ninja like Shisui. Also a common ninja obtained from passive pack source.

- Kakashi Beheading Sword (BT): Can be a nice subdps for arena. Allow you to transfer debuff and reduce opponent nin/res. He c*so scale and do few standard attack every round.

Buy or Not?: Nope. Too many option available for arena ninja. He is not that special, so doesn't deserve our ninja points and lineup slot. Also a common ninja from passive pack source.

- Lee Gate of Joy (BT): A nice blitz ninja for arena and PVE. You c*e him as a focus or subdps.

Buy or Not?: Maybe? but don't rush. If only for PVE fight, it is not really mandatory. Maybe winning arena (e.g. duel arena, ranked arena) can be useful if you don't have better ninjas. As a note, it can be better to save ninja points for other ninja that will benefit you more (e.g. to buy support for real PVP team).

- Gengetsu Hozuki (BT): I heard he is useful for arena. His clone is useful to tank opponent which target the front line or lowest hp.

Buy or Not?: Nope, he is too expensive with 2600 point per frag. You can get 30 frag of him from Edo Tensei treasure, but you still need 150. Furthermore, his frag can be obtained passively from Infinite Illusion pack.

- Kabuto Snake Cloak (BT): Useful to support obito rage and edo tensei units (e.g. Madara ETR). He is able to self damage obito to make him goes into rage mode faster, and a useful mystery to revive. His passive BT is nice to support edo tensei scaling from mystery and provide crit buff.

Buy or Not?: Yes, if you are planning to use Obito rage or Madara ETR. Although he is expensive, he is really useful for current meta ninjas.

- Zabuza Edo Tensei (BT): Another nice ninja for arena, which can be used for focus or subdps. Also, I think he has potential to be used in Infinite Illusion 3, better than Lee Gate of Joy. His stat scaling is more relevant than Lee.

Buy or Not?: Maybe? but be sure to obtain him from GNW treasure first, because he is very expensive. Another reason is because a good arena ninja is not a good reason to buy a ninja.

- Zetsu (BT): A classic troll ninja, typically used in arena. He is used to randomly clone unit in your team.

Buy or Not?: If using him solely for arena, nope. There are too many choice for arena ninja, and he may not be an option because he is expensive (as expensive as zabuza edo tensei). Also, make sure to obtain him from GNW treasure first.

- Pakura Edo Tensei (BT): Another nice support for obito rage. Can give him extra chase or increase standard attack and chase damage. Also, can be used as debuffer: she inflict irremovable ignite, suppress immune with mystery Y, and a chance of immobile.

Buy or Not?: Yes, if you are planning to use Obito rage. He is much cheaper than other support like Kabuto snake cloak, and also obtainable via treasure (save you more ninja points).

That's it, thank you for reading...