Could be boring with horses. Not so boring in GTA:O though.
You get a number of different kinds of missions from stealing a car from a car meet, to stealing it from a race, to stealing a car fitted with a bomb that you have to keep driving fast enough until it's disarmed like that movie Speed.
You get to steal high-end cars that you've probably never driven before which is interesting
You can be griefed by other players but unlike with other missions or sales it's fairer because you're in a fast car (if you're doing it right you only steal high end cars), so it's exciting in a kind of good way.
You get to mod the cars entirely for free before you sell them, which is interesting
You get to sell cars as part of a collection which is fun to do.. but sadly unbalanced as stealing only high end cars is the better option.
But I'm not so sure that with horses it would/could be as much fun.