I don't know what their reasons are and the invite rate here seems to be quick enough that it won't cause an issue, but IMO part of what killed Wave was Google's use of their standard user rate limiting on the product. I got access to it relatively early and it seemed really cool but by the time anyone else I knew could actually get into the service it was already pretty much dead.

Doling out exclusive invites can be useful for all sorts of reasons (including marketing/psychological), but you really need to be careful that it isn't so restricted that it causes your product (assuming it is at all 'social') to be stillborn.