Against it but honestly I don't care much if others do it.
Most of the time result in ghost subscribers that aren't interested in your content, other times is people that unsub from you a few days later as if you wouldn't notice.
The times I've participated years ago, I've done it mostly as an obligation than actually liking a series that I'll continue reading.
Still, I'd like to mention that one thing is when you know, participate in these threads of sub-x-sub, an another completely different thing is when someone approaches you in your DMs and ask you to sub to them just because they subbed to you (Not because they liked your stuff but because they wanted to generate a sense of compromise), the first kind I usually don't participate, the second happened several times but I've learned to say no