Quote from Badprenup
That is some serious faulty logic. I would never use a Pig in a mine because even with the Carrot on a Stick, Pig riding has always been fairly useless to me. But if there is room I would totally use a Horse in a cave. Being able to jump places mobs can't reach, or over lava or any number of things. That just sounds useful.
Besides, very rarely do I ever use Minecarts in an actual mine, as some of the most efficient mine setups are so efficient that you don't need to even bother with a Minecart for transporting items. While I agree that Horse carts would not make the Minecart useless, your logic is broken.
As for the idea itself, It's neat, but I really don't see too much of a need for it.
You're right. This is much more for multiplayer. Or they can improve the villager AI.(Ride a horse?LOL)