The only real advantage you gain from a salt these days is that a precompiled database of hashes can't be used against you. Whether it be from someone's personal collection or one of those web-based hash sites.

Generating a rainbow table for each user isn't that much more difficult or time-consuming than having a single rainbow table. Processing power is cheap and easy to come by these days. A small botnet can be rented to generate rainbow tables at a faster rate than most supercomputers.