"Why bother correcting them...if you can understand it enough to tell them what they've done wrong?" So if I can't understand what they said but can still correct their spelling it's okay? This is sort of flawed in execution.
You let people know if they made a mistake so they don't make it again or to help improve their grammar/spelling. It isn't discourteous to point out when someone has made an error. It's only discourteous when you call them a retard or imbecile for it. lol :0