I own a bunch of bikes, including a MTB with slicks (aka "road tires"). Here's a few thoughts that come to mind:
MTB w/slicks advantages:
1. A flat bar is really good for maneuverability, especially if you ride places that have lots of obstacles.
2. The more upright position allows you to keep your head up a little higher which may help you see further up the road
3. Good MTBs are built tough
MTB w/slicks disadvantages:
1. Since you're sitting a little more upright , you aren't likely to be as aerodynamic.
2. Some MTBs can be heavier than road bikes (though depending on your price range, you can find some pretty lightweight hardtail MTBs)
3. There are less hand positions on an MTB which may make it less comfortable on longer rides (you can put bar ends on your MTB to get more hand positions)