I'd say there's some good to this! Let's keep our head up shall we?
The definite bad side of doing sub4sub systems is that your channel will definitely show that you've used some sort of boosting service towards your numbers, and it makes it clear to users that you don't care about your growing community and actual subscribers. This can cause loss of interest from users, and in the long run really doesn't help your channel at all. I'm starting this for beginners, as why use a service such as this when you already have the numbers.
For advanced and popular YouTubers, I could see this as a possible revenue boosting service, which being popular already on YouTube, can help boost your standing in the SEO ranks and possibly help drive new users to your content? I might be going off on a limb here but I think this topic has a lot of controversy and needs to be discussed in detail.