You are either very new to internet forums or very naive
Of course they are going to be used for some kind of forum disruption, one account is all you need if you want to contribute positively. Of course, I'm not counting here the cases when someone forgot his user and/or password, then created new one instead of recovering the old one, such things are rarely a problem.
I am neither. Please tell me in what way Johnny Groove's supposed double account disrupted the forum or your experience on it. Or other harmless double accounts as well. They rarely do, and when they do they should be banned for forum disruptions, not for being a double account. For instance, say I created another account from my laptop and made just one post with it, something 'Federer in 2' in a Federer WWW thread and then never used it again. Would this make me a very bizarre individual? Yes, probably. Does thing bring the forum into disrupt in any way? No. Would the ensuing 3 month ban if I got caught have any rational justification? No.
Even for previously permabanned accounts, it still makes no sense. If a previously banned poster comes back in another account as a law abiding positive poster, it's absurd to ban him just because he's a DA. In short, banning solely on grounds of being a DA ie treating all double accounts equally is an inane system and one that actually favors malicious people/posters over naive yet not badly intentioned ones.