I take it the recently reported issue of an entire country (Dubai?) being blocked from editing Wikipedia articles due to the abuse of a single user of the single IP address behind which much of the country was NAT'd, isn't an issue of this then?
This is a problem for people running service out on the Internet. It's very difficult to hold an individual accountable as they often can force themselves to get another DHCP lease to evade bans. After finding an abusive user, your only options are to try and deal with the abuse, or email logs to an ISP's abuse@ and hope that they might take some kind of action. abuse@ departments don't want that job. Ident originally provided that when people used large multiuser machines, and a surprising number of services still today at least try and use ident even though it's use has been seriously limited since the mid 1990's. Why don't ISP's transparently catch ident requests back into their network and provide some kind of identifier (it doesn't have to be their username), that can be used to track abusers? X-Forwarded-For: is great if the protocols you're thinking of are HTTP, it's less useful for tracking down abuse in other protocols (eg tracking down spammers using SMTP to send mail).