I would say I am a casual mountain biker, with a regular 40+hr job and two young kids. Sometimes it is very hard to escape the schedule and the kids to put in extra mountain biking milage to get better.
Middle of last year I purchased my first ever road bike and started to commute to work about 2/3 days a week (45km/day). I try to have easy days and then hard sprint days, averaging ~33km/hr. In October I did the 250km Fitz and finished pretty comfortably.
I have realised lately that I do not love riding on the road but neither do I hate it. To me it just seams not a technical challenge (apart from races/crits).
I almost treat it like a gym session. Not that I have ever used a gym.
However, what I do love is the extra fitness it gives me. Every time I go out and hit the trail on my mountain bikes it absolutely is amazing, especially how much speed you can carry all round. I think the more fit you are the double the fun you have mountain biking, and the more focus you can put in technical skills. Instead of slowly cooking at the end of a 30km trail I am still blazing along and still having an awesome time compared to before not really enjoying it and wanting to finish.
So in summary. TLDR; if you are a casual mtb rider or above I would highly recommend getting a road bike. As per most of the suggestions in this thread.
Even if you have to also get and wear lycra.