I have a horse that was tender on hard surfaces on his front feet, so, I had to balance some of that weight to his hind end, problem solved. He is barefoot all the way around, as are all of my horses.
Horses do carry most of their weight on their front feet, however, the hind end is designed for carrying that weight when in motion. Personally, if I were to shoe my horses, they would be shod all the way around, since ideally we are balancing 25% of the weight on each foot equally, if not more so on the hind legs. As it is now, they are all fine barefoot, so I will save myself some money. If I had no problem keeping my horse on the forehand for riding, I would shoe only the fronts. Or, if I had a pasture pet that needed shoes for physical reasons, I would shoe the front per the farriers suggestion if that horse would be using the "staying legs" only instead of the "travelling legs".
I know many people shoe only the fronts, I have found it much more effective to instead get the weight on the hind legs. Thats just my opinion.