I get you in a sense, I use the multi rig in two different styles for most of my fishing at lengths normally between 7-11 inches, this only varies when I know I am in deeper silt (where I use a light lead and heli set up to counter it on the most part as you say). I normally use 8 or 9 inch hooklengths, only going shorter for stalking with heavy leads at close range.
Don't think you can say there is no need at all to adjust rig length though, that's a pretty definitive statement. Fishing a 1ft hooklength on a small spot at close range would be more ineffective than a short 6 inch hooklength. And before you say anything, I've witnessed the difference with my own eyes at hooking potential, the shorter hooklength converts far more times in this situation. But I agree with you that changing the lead set up will help combat weed or deep silt more than just increasing hook length size.
Be interesting to get you take on zigs and floater fishing hooklength sizes.