In my experience, the majority of hardtail mountain bikes and short travel full suspensions are used both on and off road. Only the truly specialty machines, whose riders have multiple bikes will be only used off road.
It's anecdotal, but out of my friends and MTB cycling acquaintances, if you have multiple bikes, you generally have a hardtail, which is used for commuting, casual neighborhood use and light trail use, a fully which is your primary off road bike, and a road bike which is used for sport road riding.
If you only have one bike, you use it for everything, to include commuting/neighborhood use.
If you do not identify as a "cyclist", and yet ride a bike with any regularity, it is likely a basic mountain bike, and it is likely used primarily on the road and in the neighborhood, because there is the perception that mountain bikes are stronger, safer, more stable, more comfortable, and have better traction. People use mountain bikes on pavement because they have them, because it is convenient, and because they are comfortable. But mostly, because that is what they have, and most people don't see a need for more than one bike.
Even hard core roadies tend to have a casual use bike which, if it's not a mountain bike, is a hybrid based on designs evolved from converted mountain bikes.
It is a rare cyclist who only ever uses their bikes exactly as the designer intended them to be used.
