Will point out here that Ted Williams pointed out long ago that an "upper cut" swing works well because it matches the descending nature of a pitched ball. In essence, it isn't really an "upper cut," but a swing that is simply on plane.
If Dan Quisenberry throws a ball coming in parallel to the ground (with no drop. so we'll discount gravity here for a second), then a swing parallel to the ground ("on plane") keeps the bat in the hitting zone for the longest possible time. It is, simply, more efficient.
Ditto an "upper cut" swing vs. a descending pitch. I'm not going to hazard the math to determine the angle of attack of the average MLB pitched ball, but I'm betting the "upper cut" nature of the "fly ball" swing approximates its inverse, or slightly exceeds it.