It's fine to sell out a developer conference if there is high demand by developers. The problem with I/O, is that there are some who go, even though they aren't core Google platform developers.

I consistently hear the key reason to go to WWDC, is that a good portion of the Apple Engineers take the week off, and go there to answer, one-on-one, questions that external developers have regarding APIs and use cases. Quite often, the person answering the question, is the Apple Engineer who wrote that API in the first place.