Well, yes, variance is the big thing. Even ignoring obvious fakes, the common argument, that brand products and cheap clones are probably made in the same factory, doesn't really hold water. They might very well come from the same assembly line, but the difference is in QA. Because there's always some copy-to-copy variation, it makes total business sense to make one hundred copies of a part, sell the top 10 to a big brand name with stringent QA criteria, and the rest to progressively less scrupulous companies.

Edit: Oh, you mentioned this as well in your edit.