There are a bunch of pieces, usually regular blocks that you won't really need again. I think the kids find it exciting to get some of them, and it won't hurt your future teams to let them have those pieces. I would definitely save any bars, gears or technic pieces for future teams. This year's spinner and strength machine are gold mines for those pieces!
I guess it really depends on if you will be coaching again. If you are, it is a team robot- not yours, but an FLL bot used by any team members of the future. If you're not, then it's not as easy a decision, but you said you were, so that's that with the machine.
Good luck fielding any questions from parents who believe they purchased a NXT machine ;)