"We can't legally install a 40A general-use circuit. "

Not all circuits are general use.

While most larger loads ARE 240 V, there are all sorts of weird things hanging around.

I've seen 30A single poles used all the time usually feeding L5-30R's. We used them extensively for larger computer servers. I've got one (I didn't install it) feeding my airplane hangar and I've seen them used for boat/mobile home hookups.

You're right, I've never seen a 40A single pole used to my recollection.

The only place I have seen 125V circuits above 30A is shore power for marine service. 125V, 50A was at one time fairly common.

I was looking for a partial enumeration of the "all sorts of weird things." :)

boats and RVs... didn't think of those... Airplane hangar? Is it feeding lights and one or more receptacles directly? Is that legal?

That's a BIG computer server!

cool! i dont know what it would be used for, but it is different. You could always use it to control a qo 2 space panel wita 2 20a berakers and recs, but you could just use a double pole cb with 12-3G

Racks of communications gear sometimes get up well past 30 amps @120 V.

They are not plugged, but hard wired.

Additional over-current protection is built into the rack of equipment.

While running a multi-wire circuit could be used, it adds additional issues with leakage currents between the equipment running on the separate legs.

It is easier to just keep everything on a single 120 V leg.

A couple of years ago I put in an off-grid solar power system for my sister. The inverter produces 4000W continuous and 8000W surge of 120V power. That is, it's a single pole system and no 240V is available anywhere in the system.

The AC panel "main breaker" is 70A. The breaker feeding her barn/utility building is 50A, the feed to the guest cottage is 40A, and the feed to the pump house is 25A -- all single pole. Had to order them on the 'net since the big box stores and electrical supply houses I checked didn't stock anything larger than 30A.

Griswold Family Christmas lighting project? :-)

There are plugs and receptacles available in very large ratings that are seldom seen in a residential setting.

The airplane hangar has a separate circuit for the lighting. The 30A circuit feeds a single L5-30R. I don't know the intended use, but that's what the hangar comes with for power. I've got a small breakout box that plugs into it that I run the block heater and a few tools off of.