100% agreed - this would certainly undermine the movement. The open question is whether it would do so more or less than the loss of half a billion dollars held by the community. I'm not sure what the answer is, but shouldn't every option be on the table?
Also, that makes it more attractive to act maliciously, as an exchange. Either you make off with your stolen BTC (win), or the community fixes things for you (not really a loss).
What would help is some equivalent of FDIC. A group of Bitcoin "banks" that handle your deposits, with some pro-BTC group guaranteeing your deposit up to 100 BTC or something. Getting the insurance would of course require all sorts of intense auditing and oversight. And somehow, someone's gotta pay for it all (perhaps the same group of Bitcoin companies pay in). But that's... very far removed from the current state of affairs.