I thought the same thing. I'd expect the developer who is thoughtful to catch client-side errors would also deploy their code minified and that JS errors are infamously unidentifiable by message alone (unexpected reference to undefined, anyone?).

I think this could be helpful given some savvy javascript written expressly to be parsed on error - but the overhead that may bring on doesn't seem worth it. And with enough users you may have a deluge of data to sift through.