Many people have a harder time using their imagination in certain ways. And, along with us using *our* imagination well, our job is to help the client manage theirs more.

So, what I've seen is that if I make the design look too finished before it's ready, they will SEE it as finished, and not want to explore other possibilities.

On the other hand, I might *want* to use, say, color, to help their imagination go in certain directions where a B&W sketch might not take them.

As long as the wireframe did not look too finished, it might actually help them spark their imagination in good ways.