There actually is a point to parallelizing cracking across users. Consider: You're testing a specific password, say, "2398fje#f", and want to see if anyone is u

There actually is a point to parallelizing cracking across users. Consider: You're testing a specific password, say, "2398fje#f", and want to see if anyone is using that password. If there are no salts, you can hash this, then look it up in a hash table of password hash to user (or, to get more fancy, you can do things like performing a bloom filter lookup on the GPU, etc). If there are salts, however, you must hash it multiple times - potentially once for each user. This applies even if there is only one user using the password in question. So the difficulty of the search is no longer proportional to the number of passwords tried, but rather proportional to the product of the number of users and number of passwords - a rather large increase!