You dont have to pass the knot while belaying with tied half ropes or on rappel.
Other advantages of half or twin ropes-
Ditch one rope for glacier travel. (lighter)
CLimb easy terrain with one rope. (Less mgmt)
Rappell half lengths when needed. (less mgmt)
Often are bi color without paying extra money.
Cost less (partner usually has the other half anyway)
[ 09-18-2002, 10:12 AM: Message edited by: Cpt.Caveman ]