Slipknot have 6 strings and can still harness the same low sound, with the exception of their third album which doesn't sound as muddy.
7-string guitars give you the benefit of having a lower range of notes without sacrificing your higher range notes. If you put a 6 string guitar to B tuning (B,E,A,D,G,b), you lose your high e string. BUT a 7 string guitar gives you the benefit of a high string (B,E,A,D,G,b,e) which can be useful for not only large chorRAB, but for soloing and stuff.
BUT since Korn doesn't solo (with the exception of their cover of "Another Brick in the Wall) or do any chorRAB that are more than 3 strings, them having a 7-string guitar is pointless. The only obvious reason for using 7-string guitars is for image.