I think the problem there is that the act of committing is really effortless, making it easy for people to be over-optimistic about participating in all of the proposals that they're committing to. So, if the majority of a proposal's user base is equally tied up in six other proposals that could be nearing beta, the actual level of commitment would probably end up being far less than anticipated.
It seems more appropriate in that case to restrict people to only a few commitments, with the expectation that if a site should fail, it will do so because the site concept isn't successful, not because the committed members at large took on too much at once.
At the same time though, it does put you at a huge disadvantage if the proposals that you've committed to aren't gaining a huge amount of momentum. The argument that you wouldn't be able to contribute adequately to all of the sites is largely irrelevant if only one of the proposals goes to beta after all.
A potential solution in that case would be to keep the commitment limit, but allow for "stalled" proposals to not count towards that limit based on certain activity criteria. I haven't observed Area 51 processes enough to know whether or not there's anything to reasonably determine that though, and to some degree I feel like it may be more trouble than it's worth even if there was (without knowing this is a common problem).