I just came here to say the same thing. I remember, way back when, that this felt like the most natural thing in the world: type my password once, and then copy and paste what I just wrote to save myself the effort of typing it again. I eventually figured out why that was a bad idea (possibly after hitting a site or two that blocked copy/paste just like this).

I think it can create problems for some password managers, too: there are at least a few options out there that aren't based on browser extensions at all, so copy/paste is the expected interface.