It is certainly unwise for most people to hold that many bitcoins on MtGox, imho.
Mark has stated 98% of their funds are in offline wallets, in safe vaults etc.
Many of the transactions in this block could be going into those.
I don't think it's unwise, as the risk of total loss doesn't exist.
In fact their system is more theft proof than anything I've come across in terms of currencies and commodities.
1. There is a 100btc per day limit on every account
2. Even if you found a perfect way to rob them of their online wallets, you only get 2% of their funds
Unless you can get him to reveal the details of the encrypted memory sticks etc & their locations at gunpoint, they will be safe forever.