If you go into a 'Mart or a sporting goods store 90% of what you see is MTB's. MTB's are to the cycling public today what "10 speeds" used to be in the 70's and early 80's. That's why so many people commute with them, they're just plain more common.
Personally, I ride a road bike most of the year and an MTB in the winter. Ideally, I'd have a cx bike. A couple of differences of opinion I have from other posters:
1. I prefer the road bike riding position. IMHO it puts less strain on your spine for longer rides because some of your weight is carried by your flexed arms. Your arms and legs absorb more of the road shock in that position as opposed having all your weight, from head to waist, carried on your spine. There's also nothing particularly comfortable about sitting upright and struggling against a headwind.
The lower center of gravity helps me feel more at one with the bike. I feel less stable on an MTB after a summer of riding on road bike.
Drop handle bars let you vary your hand position, which = more comfort
2. Not all road bikes are "racing bikes". There are road bikes designed specifically with touring in mind and they have room for wider tires and fenders, mounts for racks, etc.
I used to worry about potholes, glass and just plain bad roads but it's really not been a problem. I've hit a few potholes pretty hard without any consequences. There are tons of choices for tires in the 700c size. Some are very puncture resistant. Bad roads have been around a lot longer than MTBs. MTBs were developed for offroad use. The fragility of road bikes for their intended purpose, - riding on roads, is often exaggerated.
Here's what I do agree with: Good road bikes are way more expensive than good MTBs. Road bikes are a niche market. Quality entry level road bikes tend to be geared more to the beginning racer or racer wannabe than they do commuters. A good commuting road bike may cost you even more unless you go the very common route of getting a 70's or 80's road bike (with steel frame, room for fenders, mounts for racks, etc.).
The young people who really live on their bikes around here (in the city) tend to use old road bikes or Surlys.