I don't really get why: I mean it's not that difficult to figure out that everyone at one of these conferences is going to need more than the average amount of bandwidth. That they're going to be tweeting, facebooking, foursquaring, groupmeing, emailing and browsing non-stop. And if you don't have adequate WiFi, they're all going to take to the interwebs to bitch about it.
The solution is pretty easy: install a couple of extra WiFi connections. They're not that expensive (especially if we're talking about one of those $1,000+ conferences. And both they and the free ones can likely find some company willing to sponsor the extra WiFi in return for a few free passes.)
That way the chatter is going to be all about what's going on at your conference. Not about how they can't log into their email.
It's a pretty minor upgrade, but I promise you it'll pay off in spades.