A very good point about the friction in the garden-variety pulleys available from Home Depot and similar places. In my experience, though, these generally do
not have plain bearings, at least not in the sense of having oilite or other bronze bearings; they generally have no bearings at all -- just a pin through the hole in the sheave, rubbing its merry way to a great deal of friction.
Of course, if the sheave is made of a low-friction material, such as acetal, it can serve as its own bearing to some degree. I don't know what the implications would be for how much weight it could handle ...