3G/4G networks become extremely unreliable at events like this. However, practically nobody is on the 2G network. Yes, it is slow. But when all you need to do is push out SMS messages to meet up with friends in person, it is seriously a life saver.
Meanwhile, the remaining 2G spectrum can be re-purposed to LTE, which offers a vastly increased amount of data transferred per second per Hz, thereby increasing the amount of bandwidth available to everyone.
An idle network with dedicated spectrum is wasted spectrum. It's handy in edge cases like this, but only until everyone else figures out your secret. Then it gets instantly bogged down and the carrier is just squandering valuable spectrum.