Yes and no. They're not cumulative, you don't need to take them all. Theoretically you could just study up and then only take the N1 and pass your first time (though that's not really likely).
As for the studying being useful for whatever you do, that's halfway true. It will be useful, but it may cause you to prioritize things in different ways. That's why I'm currently waffling about whether to push for the N2.
Lately I've found a real groove in terms of my own studies. I've been playing text-heavy games (逆転裁判 series, Steins;Gate) and reading seinen comics (Billy Bat, おやすみプンプン and middle-school level novels (ぼくらの七日間戦争, キノの旅. Every time I find a word I don't know I put it in an Anki deck for that media type (used to be per-book/game, but the frequency of new words has been dropping) along with the context it appeared in, and generate two cards for it (J-E and E-J). My reading ability has increased dramatically in the past few months, and my "passive vocabulary" (words I can recognize and understand in reading or when someone says them, but probably have a hard time remembering if I want to use them in conversation) has increased by several thousand words, while my active vocabulary has also increased a great deal.
The thing is, they aren't words that are likely to be on the N2 test (though they probably are on the potential list for the N1). If I were to shift to cramming for the N2 it would cut into my time for this method, which is really working for me. I'd be studying vocabulary off of a list, rather than learning in context, and I know it would put a damper on my enthusiasm. I don't know if that tradeoff is worth it for me right now.
I am a little bit curious how I'd do if I just kept doing what I'm doing until the test date and then gave it a shot without formally studying vocab lists, just brushing up with grammar drills beforehand. But I'm not sure if that's worth 5500円.