There is no 'other side'. Its all two wheels, two pedals, and human power.
I do both 'only for fun' riding, aka sport riding, as well as 'gettin stuff done' riding, and for either one I might use a road or mountain style bike depending on the circumstance, and for the first, I might also use a mutant bike that doesn't fit any existing 'road' or 'mtb' category. If I had to pick the bike I ride most, it would be an urban assault trim hardtail/rigid fork GT mountain bike, which gets used for fun and work all the time.
As the number of forums show, it isn't as simple as 'road' bikes and 'mountain' bikes. Those are the two halves of the bike shop, but not the two aspects of riders.
peace,
sam