I think most people here still map for HL and therefor model overuse isn't really an issue for them. In HL2, the generic industrial theme and the same props get pretty old after a while. Seeing an explosive barrel isn't yet the same as seeing a prefabbed crate, but HL2 only exists for a year so it hasn't been around long enough to get really overused yet. Anyway, I'm not going to use props that are already widely used unless they really fit in a map, because I place high value on a distinct theme and feeling.
Of course, others can value different things and that's no problem. It's just my point of view and the reason why I create my own props and details rather than downloading some prefabs or models.
Then again, I'm also the type that wants to do it all by himself causing long development times, so my way surely isn't the best in every situation.
I like it however, learning things in a lot of different creative area's, and that's why I originally started mapping, because I like it, so, yeah...