Carrying this cab regularly will not cause the miters to separate, for a number of reasons but even if only with the strength of the glue, it would not.
The problem with a lot of production miters is they're glued up quickly and slapped together, and the glue is absorbed into the end grain as one would expect, not allowing a thick glue surface and I'm confident you're aware, modern day wood glues are stronger than the surrounding wood. I use the sizing method on my joints to ensure there is plenty of strong glue surface.
Next, the contents of this box is not a 40 lb tube amp, it's a 13.5 lb piece of rack gear, and this includes the 600 watt power amp (this is the PowerRack model).
Finally, the box doesn't rely completely on the joints for its strength, it is reinforced laterally first by the amp itself, as it is attached at each side using two #10 fasteners (4 total), and secondly by the front panel which is glued onto the 3/4"x1" wooden frame which is also glued around the inside perimeter of the cabinet.
It is unnecessary, but the cabinet could be further strengthened by adding a rear panel.
Now, there's no denying that dovetails, box joints or even a rabbet-dado is a stronger joint, however, dovetails and box joints are completely unnecessary nowadays due to the quality of glues available. At this point those joints aren't mechanically necessary but instead, aesthetically pleasing to many.
Like most vinyl covered cabs, although I've seen some with simple butt joints, I like to use the rabbet-dado because it's a simple joint, allows a small amount of end grain to be visible (which I personally like), and it's strong.
To your point about this being more about looks, you're right. Few purchase hardwood cabinets or pedalboards for hard road use, thus further mitigating the risk of failed joints.