1. you are proposing a Plunge Protection Team for bitcoin? That conspiracy theorists will have a field day! Seriously: you are talking about a central authority, which is not going to gain much traction in the community as it is pretty much opposite to the philosophy of bitcoin.
2. I think you are right, or at least I think it would be 'better' if there was some kind of guarantee in the case of massive hack. I think it would be a really different type of company though, think about it: if there were a security breach then users would have to be identified before they could be paid their 'compensation', that means that the users would have to be officially identified by the MtGox company, using proper ID etc. There are many MtGox users that would not like that.
As you quite often hear around here, why not set up an exchange and try to provide the protection you describe above?
