I'd like add to the dissention. Pro's use indoor trainers with wattmeters for a LOT of their training.
1.) It's safer.
2.) Weather is no longer a factor for 3-4 hour rides. No sunburns, rain, or frostbite.
3.) Daylight is no longer a factor. Ride at midnight if your body is more of an owl. Or that is when the kids are asleep.
4.) It's a highly controlled workout.
5.) Hydration and food can be place safely within arm's reach.
This is a typical problem... the position you ride on a stationary bike tend to be VERY upright, engaging quite different muscles than when you road on an actual bike outdoors. The time spent on the stationary is not completely wasted though, you still built base conditioning (although you would've been better off spending the time running to do so...).
The resistance on the stationary bike might also been set really low, which means that you can ride very fast (just like riding downhill...).
Third, stationary bikes tend to be absolutely horrible when it comes to showing ACTUAL MPH... it's entire possible, and even likely, that you showed 15MPH on the stationary and 9MPH on your road bike with exactly the same effort.