Yes, it is sad when people say that a horse will not recover and then give up on them. However, did you know that in humans, bones can take 6 weeks to heal - and that's only in children. In adults, they take more than 8 weeks, and that's in a fairly mild break. I expect an adult horse would take at least as long as an adult human, probably more as their bones are so much thicker.
In my opinion, it is cruel to have a horse stand still for that long while it heals, if it heals. Bones don't always heal properly and there can be a weak spot where it healed. This means that stress on the leg could cause it to break again. While it might be ok to walk and trot this recovered horse, cantering, galloping and jumping would be definate no-nos - they put stress on even the bones of healthy horses. We have to ask ourselves, is it fair to put an animal through the long, long healing process when the chances of complete recovery are very slim?
Also, horses that break a leg often experience problems in the leg opposite the broken one because of all the excess weight it must carry. I know exactly how you feel, but sometimes the kindest thing is to put the horse to sleep rather than cause so much more suffering. I do wish there was more we could do for these horses.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare