David Weaver
..it's harder to get a chisel driven straight down out of the cut, or pushed forward and back to free.

I have a champion socket mortise chisel - it's 1/4th wide, and already 3/8ths tall just at the tip - and then tapers thicker in height. It is almost a dandy cabinetmaker's mortise chisel for bevel riding except....

...the sides are parallel. It's horrid. I will fix it. (the other interesting thing about this american socket mortise chisel is that whoever had it before me decided that they should round over the top of the chisel. I'm supposing that this was done by a user based on bruising mortises or not liking the action deep in the cut. It could've come that way, but it's been used enough that such a thing can't be guessed at. Interesting, too, is the factory did chose to bevel the top corners of the chisels, similar to the round over for the law chisels on Joel's site. They must've thought it would help).

I also had a miyanaga mortise chisel (6mm, pretty much square in cross section) that didn't have enough taper and though I got it used, they are fairly expensive. I didn't have the heart to grind it back and potentially ruin the value of the set. In making face frames for my kitchen, I managed to break it at the lamination just in normal work. I sold the remainder of the set noting that they'd be better corrected. Someone still bought them.

Back to your original point. The chisels just seem to work better if ridden down on the bevel, and some of that is probably because they're scraping the sides as they travel.