Other than the very basic ninjas such as an early gnw 1010, you are better off spending it all on power as a f2p or low spender; if your wish is to be competitive.

The good event ninjas cost so much, that by the time you save up for them as a f2p over the course of 4 - 6 months, you'd be very behind in power. While your new shiny ninja can keep you competitive for a short while despite the power gap; when the meta shifts, you are no longer competitive, with no chance of ever being competitive again without going whale mode.

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If your goal is just to have fun, then by all means, collect all the ninjas you want.