There's a use case for MTB and hybrid but it's not what I would consider optimal for someone looking to get exercise or travel distances of more than a several miles at a time on a well-paved surface. Lots of people bike for a bit of fun, a short commute, or a trip to the store, which is another story.
In the end the best bike is the one you actually want to get out and ride, and for many people that won't be a road bike even if they bike on the road.