I personally like barbed wire better than just smooth wire. At least with barbed wire if a horse ever rubs up against it they will most likely think "Ow, that hurts." Then, hopefully, they will learn that the wire will bite back. Smooth wire on the other hand, they can rub up against it and nothing will happen.
Barbed wire definitely isn't my favorite. But, horses can hurt themselves in the strangest way. I knew a horse who was extremely hard to catch and was running around his paddock. Well, there was a bolt (about 1 inch in diameter) sticking out directly parallel to the fence and wasn't very far off of the fence (it was holding the gate in place). No one though a horse could possibly get hurt on that so it was just left alone (These fences were probably the nicest wooden fences I have ever seen). The horse somehow managed to run into that bolt and impale himself in the chest.
My mom ended up shoving her hand into his chest and squeezing the artery that had been his shut. He wasn't even our horse, she was just the barn manager. Luckily, it healed up so there wasn't even a scar. But, it just goes to show that horses can hurt themselves on even the nicest fencing.
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie