The reason the belt retractor fails is because the nylon spool breaks. It is very thin and is just not up to the task. If you look on the floor under the retractor mechanism you will find pieces of thin, white plastic. Once the spool starts to break, then the belt rubs on the metal housing (rather than riding in the plastic spool) and the belt retraction is much slower (because there is a lot more friction). Lubrication will help a little but the piece is going to fail, no doubt about it.
Poor quality piece, that spool. Kind of reminds me of the ashtray door in the older Mustangs!
Chris