The other guitarist in a band I played in had one.
Theoretically, it could provide a "best of both worlds" type thing. And there are probably a few owners who think that, but most people end up just going back to the regular full backed 412.
Also, this doesn't get said a lot, although most manuals for speakers of any kind and cabs say it somewhere, speakers aren't supposed to be directly against a wall. Usually, you want at least 3 feet.
Probably like, 75%-80% of cab users park theirs against the wall.
The reason is obvious, the full rig thing takes up space.
It's usually not that big a deal unless you are a huge snob, but if you have an open back or semi open back and your cab is against a wall what is going to happen is the sound from the open portion is going to "leak" a lot more than the sound from the closed portion. There is going to be a lot more sound bouncing off the wall than a closed back cab. When you mic up the road king ones with the semi open back they are pretty much meant to be somewhere in the middle of the room. You are going to get a lot more sound building up between the wall and the back of the cab running it at anything more than whisper quiet volume than you would with a fully closed back. The build up won't be symmetrical either.