It's a good question, from my perspective it is historical, (that is how PRSh first did it). Now as to why he first did it, maybe someone else can answer. He didn't finally agree to a 4 control layout until he was building the first SC's in 2000. While it is true that it cuts down on options, it also cuts down on complexity. It's pretty simple to operate the 1-1 controls and I don't think you loose much in terms of tone. Evidently DG and BM did think it was worth adding one more volume knob, and I can see the point, but I still don't use two different volumes in songs much (maybe others do).