The problem with models is collision/clipping. I could make a dumpster model, but for player model to get into it, you'd have to clip it precisely anyway. It's simplier to just create such things in bsp.
Edit: Unless you want to clip it like a box inside and outside. Then a model would do, and it would be actually easier/faster/nicer to make model in, say, Blender, than in bsp. But then again there are problems with texturing models and exporting them; all in all it may just not be worth it.