Well, you don't do barrel rolls and vertical loops in a car or a commercial jet, either. I like the idea of coasters being equipped with the roller/retractable seat belts, because that would make it much easier to have the correct length, and people could not leave the seat belt so ultra loose as to have no point. The reason that they attach to the harness, as far I have seen however, is as another safety measure: if for some reason the harness were to unlock, it can be held closed (to a degree) by the seatbelt. If the belt were not attached to it, it would simply fly open, voila.
-------------
PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/