If a client requires a copy of another site, then no, his requirements don't supplant everything. If you know exactly what you want, you don't need a designer, and hiring one will just piss both of you off.
A 'designer' by definition is someone who designs — comes up with a plan — not someone who implements (which is completely secondary). And frankly, the designer is the expert here, not the client. So all a client should do is fill the brief, and give specific feedback when asked (not say things like "I like that logo because it looks fresh"). That's it.
The client should trust the designer to make the right decisions based on his requirements. And going on the same note, a good designer will ask the right questions and pull a client's requirements out of him. This, you're right about, yes. But clients WANT to guide the designer, so YOU might not want to hold hands all day, but most clients do, and that's why we reach the situation in the first paragraph : http://theoatmeal.com/comics/design_hell
This post is from the point-of-view of a designer, so the client is (obviously) the bad guy. I'm not defending either; just stating why your comment is wrong.