Lameness

I've been working in the equine hospital this week (12 hour shifts plus on-call during the night is a hard slog) and it seems that most of the cases that are in are because of some degree of lameness, especially younger horses.

In your experience how lame can a horse be and at the same time stil have a career doing something other than eating grass? A few of these hroses are never going to race, and I'd like to know there's something they can be put to.