It kind of looks like an out of place wart on most bike designs, even the ones hanging off the swing arm like you mention, and also on the Origin and most dual sport/ADV bikes it's this big thing hanging off the end. Hence the popularity of "tail tidy" kits in the aftermarket.
I imagine this is a leftover design element from the days of needing a largish incandescent bulb housing for the brake light, as well as designers wanting to taper the rear of the bike as much as possible behind the seat. Since we now have super bright, ultra thin LED lights of wildly varying shapes available to work with, it's probably about time the plate moved up to the rear light area, with a light bar surrounding it, along with the signal lights. Sure, the rear of the bike might not taper as tightly in design, but then only a light, thin fender would be needed to stop spray and could add to the design. These Can-Am designs were inspired by a snow owl, well, most birds have long thin, tapering tail feathers (without a license plate hanging off the end;). The rear light of the Pulse is actually about the same width already as the license plate mount. Imagine if they'd designed the plate mount up there with a light bar above and below, and vertical orang signal lights either side.