Some people are capable of more, but for whatever reason have fallen into a rut of uselessness. For those people, a warning can help and they might even go on to become productive commenters. I've even made good friends out of people I nearly wrote off in the beginning. If he/she's not one of those people, the warning will do nothing, the hellban will come soon enough and my comment will have done no harm. I like to see people have a chance to better themselves, even if they probably won't take it.