Rarely.
I think I fought a dragon once in 2e, and in 3.x dragons were wimps.
Yes, you read that right. Wimps. They weren't designed as solos and didn't have the AC needed to take on a whole party. (They did have high saves and hp, of course.) Also, other than their breath weapon (which you could deal with using one spell*) their damage options were actually pretty lame.
They're also rather difficult to put into an adventure. In my case, it's worse, as I don't like the way they're presented in many adventures anyway. (I like them being loners. Not just solos, but actual loners. The only "minions" (non-game term) that are acceptable are its hatchlings and the only accept ally is its mate.)
The Hobbit actually put the dragon in a good place. The dragon wasn't the villain, it was simply an obstacle to remove in order to get to the treasure. The way I run games though, it's not treasure-hunting, so PCs are more likely to run away if they think they're going to encounter a dragon.
*Assumptions, of course: No surprise, it's a chromatic dragon with no alternate breath weapon, and of course the dragon's pathetic caster level to CR ratio meant it had next to no chance of successfully dispelling abjurations.