I don't know about "oft-cited", because I haven't actually seen any cited. But distinguishing these standard electric assist bicycle from the "electric trail bikes" that have been round for a while, I can guess some of the advantages compared to an unpowered MTB:
For the Rider/Purchaser (not necessarily the same person)
- ability to ride places they couldn't otherwise get to
- more ride options with a fitter partner ("together time" in many couples)
- unique bike. For a while at least, no-one else is like to have one of these.
- easier riding. You can stay unfit, but still go mountain biking
- for off road touring, ability to carry/tow more gear
For the manufacturer
- more profitable, because it's a small market niche
- less competition
- opportunity to "lock in" retailers by requiring brand-specific parts and test equipment
- easier vertical integration via the same mechanism (more of the parts are from one manufacturer.
I suspect the market for basic, obvious MTeB's is more couples and families, allowing them to go out for a ride together that's more interesting than just easy laps around a well-formed trail in a park.
Of course, none of this is actually new. The first power assist mountain bike I saw had a bunch of lead-acid batteries bolted to an early mountain bike, and that would have been before 1990. I've been seeing what look like reasonably capable MTeB's in catalogues and online for quite a few years.
The slow change I'm seeing is that there are more "MTB parks" and so on with very easy trails, so it's more realistic for someone who rides with assist on the road to also ride the easier trails. The technology is also there now to conceal the motor to a large extend, reducing the reluctance of unfit people to buy one and go out riding with their fitter mates. At some point that tech crosses over with outright cheating in races, which we're also seeing more of now.
What we have been seeing for a while is "electric trail bikes" sold as "electric mountain bike", with the huge benefit over a petrol "trail bike" is that the former doesn't need registration and can often be ridden in areas where motor vehicles are banned. This is also why they're controversial with many MTB people, as they allow silent, very fast, and unexpectedly heavy bikes to use MTB tracks.
There are two differences between those and power assisted bicycles though. Bicycles in most countries are limited to 200W or 250W, where trail bikes are usually 1kW or more. The second is the key "assist" word. The bicycle can be ridden without power assistance, the motorbike can't be. That makes the difference between an "electric mountain bike" and a power assisted bicycle pretty simple - if it's under 250W and you can ride without the motor, it's a bicycle.
There was a case in NSW where a cop told someone on a "pedal scooter" (looks like a motor scooter, has pedals sticking out the sides) to turn the motor off and pedal it a few hundred metres, which the rider was unable to do. Ticket issued, court case lost.