Cons:
- You need a stiffer fork
- If they aren't in tip-top condition they rub when you aren't even using them
- Weigh more
- Usually a bit trickier to maintain
Pros:
- You can fit really fat tyres if that's your bag. But on a road bike? How fat do you want?
- They feel nice to use (but rim brakes will stop the bike just as well)
So personally I don't like them on road bikes.