We went over this whole thing with Mir, too...
Larry is right, particularly about the contamination issue.
However, Brian has a good point about ease of access. Someday, it would make sense to put certain sensitive science satellites in orbits that are far enough away from a manned space facility to avoid contamination (a good number of miles away), but still reachable by some form of space taxi.
This would also be true of zero-G space manufacturing facilities, which typically do not work well attached to a very jiggly manned space station.