The thing is, that head is pretty large and it would be awkward to sit it on top of a 1x12, not that you necessarily would. And you need the power handling ability in one speaker since you aren't splitting it up between two or four.
Not crazy to get a 412A though, although maybe not practical. Sometimes it's good to be crazy. A long time ago in the early eighties I got a JCM and the cab I had at the time was a 1x12. After about a week I went out and got a 412 slant cab (w/ G12k-85s) to make a half-stack and was very impressed with the increased woompf and bigness. Probably the closed back had a bit to do with that, not to mention those speakers with their solid low end. It was like a Harley compared to a Honda, in the way you could
feel the notes physically. I was playing out once a week so I was able to turn it up more in those situations. And it looked real cool. I was glad I had it and used it until a couple of years ago. So it's not totally crazy.
The impracticality is that it takes up space and is harder to move around. And as Surf said, the additional cost. Although if you love the sound the cost is always worth it. And if you don't need to move it....
FWIW, I've got three 2x12 cabs from Avatar nowadays, each with a different pair of speakers. G12Cs, G12K-100s, and a Blue Dog/G12H-30 split, depending on the mood and the amp. I'm liking the G12K-100s the best right now for the JMD because they have such clarity and good balance, and rock solid low end. Of course, one of those can more than handle anything the JMD50 puts out with ease. But they're all nice tones. The Lead 80 isn't a bad one for a 1x12 either.