it's not even contributions, but that other people might start asking for features, discuss direction independently (which is fine, but jblow has been on the record saying that he doesn't want even the distraction of such).
The current idea of doing jai closed sourced is to control the type of people who would be able to alpha test it - people who would be capable of overlooking the jank, but would have feedback for fundamental issues that aren't related to polish. They would also be capable of accepting alpha level completeness of the librries, and be capable of dissecting a compiler bug from their own bug or misuse of a feature etc.
You can't get any of these level of control if the source is opened.