I think it's fine for those who does it. Maybe the algorithm or something if any of that exist, their story would benefit from that. I don't think that's a thing on Tapas, maybe.
Though, I would advise most to just, not spend their time doing that since it isn't worth much, other than perhaps maybe some find doing that stuff to an encouragement of one another or a positive aspect of the community, supporting one another through that aspect.
Then there's the rare rare rare chance that a person who sub for sub actually would have sub without the sub for sub.
I don't think it should be look down upon, 'cause I do think that encouragement is 'good', and some people will earnestly check out others writing.
Beyond that, I don't think there's much to it. The idea of a 'sub', is someone who genuinely enjoy your story and would actually like to read the next chapter when it's updated. If we're talking earnest, then it shouldn't be called 'sub for sub' in that regard, it should just be like, check out my story, I'll check out your.
My overall thought is like, I rather have 1 person whom actually enjoy my story rather than 100 whom earnestly thinks its terrible. Which is fine, I think you try to find the audience that you're writing toward and build that audience naturally overtime.
But, some people will equate the numbers of sub they have to their success or approval, so I can understand that too. Though, I would advise, that's not a healthy way to perceive things.