A rainbow table would still work if you're using one clear text per server, I just have to build the table per server. The reason salts remove rainbow tables as

A rainbow table would still work if you're using one clear text per server, I just have to build the table per server. The reason salts remove rainbow tables as a way of attack is that they're different per password, so I'd have to build an entire table just to crack one password. If I'm doing it per password there's no point in saving the table, I can stop once I've found the matching password. If I have a whole set of hashes which share the same salt though I can build a table and crack more of them at once.