Ok, with the risk of looking completely stupid, what other types of malicious users do we have around, I count spammers as bots, and we have ban evaders which in most cases are smarter than the usual newbie and in 99% cases know how to use a VPN. What others are there that can pose such a threat?? Of course, you're a moderator and you get access to a lot more data than me, I've also just read the reply theymos gave on the other topic so probably I'm having the wrong impression about this one.
Off the top of my head; Spammers, advertising bots, malware spreaders, ban evasion, and I'm sure there's plenty more. For example, even with the evil score, we have outbreaks of these types of users, where they bombard the forum with various different things. A few months back there was a lot of nuked users if anyone was paying attention to the modlog, and that was a result of these outbreaks.
The idea is, we make it as difficult as possible for these types of users to register accounts, at least without using a clean IP. If they are using a VPN, they are likely using the most popular ones, i.e the cheapest. Eventually, those VPN's will accumulate a lot of evil score, and it makes it more expensive. Now, this does absolutely hinder the process of registering for genuine users, but unfortunately its probably contributed significantly to the amount of malicious users that get through. I'm sure theymos has the numbers, and if it wasn't working, he would likely remove it.