All it really does is force spammers to search for someone's real address to spoof and harass. (They can just scan usenet, and pick on random people if they want..)
The ultimate end of this line of measure/countermeasure is code that sniffs out peoples passwords so spammers can "borrow" their accounts to send a million or so messages.
But that can be defeated with encryption. Another possible "ultimate end" for the spammer wars would be making spamming illegal, like fax spamming, and having the cops hunting down the spammers. But that can be defeated with truly anonymous markets, still assuming that encryption will be legal. To really beat spamming we probably need filters that only allow messages from inside our web of trust. Don't you agree?