A plastic pulley is actually a small plastic drum, made to clip on a carabiner. These pulleys do not have a bearing, and they simply roll on the biner, using is as an axle. Plastic pulleys work best onbiners that are symetric along their lonx axle, i.e. oval and HMS biners. The do not roll well on D shaped biners. Also, they will roll better on biners with circular cross section.
Plastic pulleys are dfinitely less efficient than "real" pulleys, because they have no bearing to reduce friction. They do, however, have much less friction than just a biner, and are much lighter than "real\" pulleys. It is a good idea for alpinists and multipitch trad climbers to have one on the harness, for emergencies that require hauling and/or a mechanical advantage system.
A better option is to have a revolver, a biner from DMM that has a tiny pulley at the point where the rope passes.
contributions to this page by: : Mica yaniv and others...