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.
I am steadily buying MtGoxBTC with my stuck MtGoxUSD (failure of MtGox to remit funds) and will continue to do so until I'm out of cash.
I've already written off the value completely in my accounts, so if (a big IF) MtGox ever recovers I will get a windfall.