Popcorn time. I was right.
Movie's over! Just part of the general PSA about tools being made by the maker for professionals at one point, and at a time where their performance mattered from an economic standpoint (mortising machines and power drills probably eliminated the need for pigstickers - to the point that they were never made in an automated process later whereas the bench chisels appear to have been switched to automated grinding setups mid1900s or a little earlier).
My point about using these also stems from a debate in the US about how the rounded top of the flat bevel is there to prevent bruising the ends of mortises. it doesn't really do much for that. In a mortise where that part of the chisel is buried (which can be pretty deep given the length of the bevel), removing the sharp corner on the top of the chisel allows pounding the chisel in (bevel down) fairly deep, then literally pulling the chisel back up just a hair (you can't break a huge chip off at the point of the cut, but if you pull the chisel up a 16th or so, you can strip a large chip from the sides of the mortise pretty easily) and rotating. The thickness of the chisel allows for a generous amount of rotation.
This is sort of crude compared to the more standard method of taking equal incremental shavings off of a mortise and has mostly to do with dealing with the ends of a mortise - this will make sense if one uses these bevel down as they'll cut at an angle equal to the bevel angle. Cutting across end grain at an angle is always easier than cutting directly across, and it preserves the ability to rotate the chisel at the bottom of the cut to sever any waste near the bottom. but it leaves you with two triangles to clear. The round part at the top of these chisels always you to move the chisel a little more easily - if it's sharp, they don't move in a deep mortise easily.
I have never read what I posted here - I figured it out and it helps illustrate why these chisels are often used little - a mortise has to be deep enough so that appreciable work is done with the rounded top of the bevel below the top of the cut. Otherwise, it doesn't do anything. The long bevel on these chisels is a product of the thickness of the chisel, which means you're really not getting into any advantages with them until the whole bevel is comfortably in a mortise.
Mine came with a crude secondary bevel on the tip of the chisel. It's my opinion after using them that the reason for that in the original laminated chisels is to eliminate riding a wrought iron bevel against wood end grain (lots of friction) so that only a small part of the chisel is in contact with the mortise. This is, again, a matter of effort and efficiency. Something that doesn't matter in sellers-jacobean woodworking, but would've been a big deal when you were doing bulk work with these instead of using a mortising machine.
The secondary bevel is crude and often not even square, I guess, because it was applied quickly by someone at a grinding wheel.
The fact that these things have to "go deep" to really get usefulness out of their ability to break out big chips bevel down (at the bottom of a mortise) means that there's not going to be that many instances where they're useful above and beyond a sash mortise type chisel or a smaller socketed chisel with a tall cross section and a little taper.
The tapered width on them also helps a lot in deeper mortises if you're being a little inaccurate.
I was only told later (when I mentioned friction) that some of the old texts talk about experimenting with oils and fats on the bevel - never tried it. If you're making a lot of beds or something where you'll have a one pass mortise that's deep, then these things suddenly become useful. if you're making cabinet mortises, probably not, and if you're making mortises that are deep and wide, it's probably far more productive to cut two narrow mortises and then remove the center with a regular chisel, or cut one and then work from left to right or right to left, or remove most of the waste with a turn screw and then pare the walls.