It's a complete mystery to me as well. I presume they think it's better to hold MtGoxFiat rather than MtGoxBTC, but if MtGox goes bankrupt I don't think it would matter much.
That's true. Exchanging a commodity you can't withdraw for a currency you can't withdraw seems like a foolish thing to do. The only reason I can think of for dumping bitcoins and getting money, any money, for them is in a bankruptcy court a judge is likely to recognise value in dollars whilst the same may not be true in bitcoins.