While it is true that a strong p2w will always best you even if you are 100% efficient, if you do for power you will be in the top 5 of your server which in turn means more free stuff such as refines and seal scrolls. The "go for power" applies to what you should do in the majority of the events. Where you hesitate to go for a ninja. As an example getting dp hidan is great and all, but you really need a team behind him or else you won't be able to use him properly.

If you go for a ninja it's because either you like him a lot (Jonin Minato for example) or it will be an end game ninja that you will use for the next x months (tentent or sailor Sakura). Going for Hanzo and then realizing you won't put him in your lineup because your prévois team is stronger is simply a waste of coupons for a f2p.

However if you really love a ninja and will make the game more fun to you then by all means get him.

This game is oriented to pvp so not having enough power will make you a punching bag for other players which ultimately will probably bore you and you will end up quitting the game. However not getting any rare ninja is also boring and can end up in a similar situation where you get bored of the game. Try to balance the 2 and inform yourself of what is coming so that you can save seals/coupons for things you really want.