I generally replace the tube on the road, but patch it and put it back in the bag when I get home. I can do a cleaner job of patching at home, plus I'm more likely to let the cement dry properly before applying the patch; on the road I put the cement on and get impatient and put the patch over it too soon.
I'm too environmentalist to just throw the tube away. The tube in my rear tire right now has 5 patches on it and no problems. I don't think I've ever had a patch leak.