Well, the Rumble Pak could be considered free, if you consider your entire purchase to be for Metroid Prime Pinball (assuming you got it with that, which unless Nintendo has started selling separately yet, is probably the case). And I guess that's how they want you to perceive it anyway, even if you could be buying the Rumble Pak and getting the game for free.
The question is: is Metroid Prime Pinball worth it? I love Metroid games but I wasn't sure if I should bother with this. The Rumble Pak sounded like a nice bonus but now it doesn't sound that great, apparently, and even if I would like it, drawing back to the original question: will more games use it at all?