Conferences to show off new startups are a good thing, but it seems like a bad plan to make the startups launch at the conference. Unless the date happens to be exactly when the startup would have launched anyway, this constraint is going to force them to launch either too early, or too late.

Launching late is very dangerous. Someone else is probably doing what you're doing, and if you launch even a couple weeks later your company could for the rest of its life be described as an X-like site, where X is your competitor's name.

Launching too early is even more dangerous. The right time to launch is as soon as you have something (a) useful and (b) not totally broken. Which means that launching early = launching something useless or broken.