I was mostly responding to the claim that Bitcoin transactions aren't reversible.
To your point: I suspect multisig doesn't help solve the problems we keep seeing with exchanges. An exchange converts between currencies, and wants to make transactions easy. A bank keeps your money safe, and wants to make transactions more difficult to that end. Unless you're a currency day trader, I don't see a reason to conflate the two.
Personally, I'm appalled by the fact that we've built a system that makes it easy for individuals to secure their own funds, and yet we're using it to entrust funds to third parties who don't have a clue. All in good time I suppose.
It would also make trades a little more difficult in that users that are slow in signing off a transaction slows the trade down. The buyer has to wait until the seller has performed an action. Could you DoS an exchange with multiple buy/sells that you don't release?