I think you'll find that changing the UQ is the more difficult way to regulate comfort. End venting and adjusting your top insulation are just so convenient and quick.
Here's an extreme example; I sometimes put my hammock up in the basement with my 0* incubator UQ, and can lay on that quite comfortably in 65* house temps with no top insulation.
Now if you're trying to lighten your UQ or make it pack smaller then a variable layer system does make a lot of sense.
I think it was Demonix who made the point once that with our bedding at home we change out our top comforters with the seasons but the mattress doesn't change.