I have only had my channel for 3 months and I already feel overwhelmed with "I subbed, sub me back". At first I thought it was like a courtesy thing. Like supporting each other and helping each other grow. It wasn't until I started learning more about it that I saw just how bad it was in hurting your ranking. Just the other day I had someone comment on my video by referencing something that is only in the first 30 seconds so I know that is all they watched. I would rather they didn't watch it at all!! When people don't watch your videos or only watch a little bit of them, it's telling youtube that your content isn't interesting enough to viewers. Honestly, I would rather have a few subscribers who love our content, than a bunch of people who couldn't care less to watch our videos. We put a lot of thought and work into the videos and I only want subscribers who appreciate that!
I have never asked for sub4sub, but they all came out of the woodwork and the amount of them is increasing everyday. The biggest issue I have right now (besides dealing with all those comments) is that many of these people have subscribed to me with the expectation that I will subscribe back so I am gaining sub4subs without wanting to or asking to. I'm not sure which ones actually like our channel, or are just expecting the sub back.
Bottom line, I have yet to see a good reason to sub4sub because the "benefits" it gives (higher subscriber count) has even bigger negatives attached to it.