You simply cannot have 500 mifi-type devices transmitting data (along with the hotel's actual WAPs) in the same room without major problems. There are only three non-overlapping channels for 2.4Ghz 802.11b/g/n, and mifi devices that I'm familiar with pick a "good" channel on startup -- they don't limit themselves just to the three non-overlapping ones.
5GHz is better, and the decreased ability to pass through walls is likely desirable here. However, your mifi is still broadcasting in 2.4GHz, and wifi clients will fall back to that if they don't like the quality of the 5GHz signal. OS X, for instance, is very finicky about using 5GHz on an SSID that broadcasts for both 2.4/5. Right now I'm approximately 6 feet away from my AP, and my laptop has switched to 2.4GHz.
Ultimately though, this whole mess is the fault of the hotel's poor implementation. If their wifi was free or low cost, and worked well, then why wouldn't everyone use it? That people are using their mifi devices says they are being ill served by the existing solution, not that they were just dying to pay AT&T more money and carry around yet another black plastic brick. By providing crappy service, these hotels have created the problem they're complaining about. Either don't do it at all (and save money / hassle to boot), or do it right.