Mostly just to prevent unnecessary pings for people that don't like them, it would say somewhere below the replying user's profile box if they were subscribed or not.

not sure if id like this bc i don't want ppl to see im subscribed to threads lol, if you want to prevent unnecessary pings you can have pinglists (on specific types of threads) or the person can just say they're subbed

Don't think this is really useful since I'm not sure who would even be subscribed to some seemingly random thread

people who are subbed to threads know they might get pings which doubles the notifications, it's not that much of a bother, I think the breach of privacy that would be is way worse ^^'
If if anything I would suggest maybe making it so when people ping you in threads you're subbed to it doesn't give you duplicate notifications it instead merges into one.