That depends what ninja. Everybody who has been playing for a few months had a chance to already obtain free nagato or hidan. Not every event-only ninja can be obtained by pure f2p, but some of them (sage naruto, nagato, dpb hidan, kazegaara come to mind) were so often and in events that give free points over time (mission dealer, time limited etc), that you could easily get them without spending a cent. It takes time and it's good to do research what ninjas are often available and focus on getting their frags. Other ninjas require either way, way more time to complete or a coupon investment (eg. mei, 1-5 hokages).

on topic:

Events give you a choice: free ninjas or free power (to some extent). Personally I like getting new ninjas, even when piecing them takes almost forever (hanzo 30/80, still a long way to go). I also pick charms whenever there's 0 ninjas that interest me and get threads from monthly. That strategy resulted in having most of available roster (guess luck with seals - 6 rares=0 duplicate - and mission dealer had a major influence in that) except new ninjas and super rares from sage treasure and further. Also got the best BA at my server for now. Downside of that are my refines, which are clearly lacking. That was my strategy and it seems to work out pretty good so far. The point is, with playing as f2p or low spender, you cant have both. Every player is given a choice what to pick and if they don't think it through, it's not everybody else's fault. Even though some events are what they are, most of them offer free stuff to choose and Im happy with that. Hope to get kage minato someday ^^