u still have to think about the other f2p/low spenders on ur server not just the big spenders. if u dont want to ever win but want to collect ninjas then sure get ninjas but u wont be able to beat any1 on ur server who went power instead. its reccomended to go power because ninjas will come to u eventualy anyway. roshi and han need 220 pulls max so even if u only do free pulls ever other day its 440 days to get them both, start adding in 10 pulls when u save the scrolls and u get them faster.

power on the other hand takes alot longer to get, just farming advanced refines for free right now(if u somehow get first in spacetime every day) will get you max 122 a month. and you need 800 + 1600 + 4000 total. that would take 52 months to get, so its alot harder to farm that power than it is to farm that ninja.

plus events give ninja fragments easily enough, right now just using stam pots u should be able to farm 10+ hidan frags from time limited shop, and there is nothing there to buy to get power(purification runes come fast enough on their own and u need the protection runes to make real use of them).

plus u get rare ninjas from free pulls, every 4 months u should pull a rare ninja per treasure roughly, so y spend coupons on those ninjas?