Why does anyone use MtGox?
The sole and only reason to buy there, is because you already have fiat stuck there and you want to get it out ASAP.

About selling, if you are optimist you can say that some people value that the risk of Mt.Gox defaulting plus the time they have to wait without their money is less than the price difference with the other exchanges.

If you are pessimist, you could say instead that they are simply stupid and do random things without really thinking about it.

Still, since there's no reason at all to send fiat there to buy BTC, yes, I agree it makes no sense for Mt.Gox to be still in business.