I think the two most important QB stats are Yards per Attempt and INT %. Completion percentage and TD percent shouldn't even be a factor in the QB rating.
TD percent is more a function of the system, as opposed to the player. A team that prefers to rush it in for a TD will have a lower count than one that uses play action passes a lot.
Counting yards per attempt AND completion percentage seems redundant. Lets take two QB's. QB1 completes 12 of 20 passes for 100 yards (100/20 = 5.0 YPA). QB2 completes 8 of 20 passes for 100 yards (100/20 = 5.0 YPA). In both cases the QB went back to pass 20 times and totalled 100 yards. Does it really matter that one did it with shorter passes and the other threw it farther? IMO completion percentage needs to be thrown out if you are using yards per attempt.