I was always under the impression that the main point of salting a password before hashing (unique or not, stored in cleartext or otherwise) was to prevent the hash from being referenced against a rainbow table of precomputed hashes. Obviously this is in the case of using md5/sha1 for your hashing.

As an example, find the md5 of a dictionary word and google it, its original value is bound to be in the first or second result. Now find the md5 of that word and a random string (aka a salt)... google and there shouldn't be any results. and even if there was due to a collision, that password wouldn't work, because it would be resalted prior to the comparison happening.