Well, copying buildings is this weird grey area that I think you could argue either way on being good or bad.
We copy Valve's props straight into our maps; is that somehow more ethical than copying brushes? Why don't we all make our own props if we have to make our own brushes?
I think of it like this: if you copy someone's writing, you're plagiarizing. And if you paraphrase their ideas and don't cite them, it's still plagiarism. So I guess in that strict definition, we are already plagiarizing Valve with our mapping. If I build my own desert themed map with the same design philosophies as Valve's, it's technically plagiarism.
Then there's this example: if I were to use a Valve building as reference, and build my own building that was basically the same with some minor scaling changes, and maybe a few awnings or windows switched around, it's okay and everyone's happy. But if I copy/paste the original building, uh oh, that would be unheard of. But the end results are essentially the same.
It's like this map: those Valve buildings conveniently fit the layout (I assume). If Noggin uses Valve buildings, OH THIS IS BAD! But if he makes his own that's the exact same scale, exact same textures, but with some tiny differences, that's okay? Why?
Personally, I don't copy/paste anything into my map because it looks lazy, and usually it doesn't fit with the map. But... I don't know if I could call it "wrong." Maybe I'd call it lame, but if you want to do it, go right ahead.