I've been on forums that had this feature, and found it by and large helpful. My general system is to not subscribe to a thread unless there's a particular reason I want to, since when I had a subscription to every thread by default, my subscription list quickly ballooned to an unreasonable amount of threads. But I'm generally more likely to care enough to reply in the future if someone did find my post interesting enough to quote it, so this strikes a nice middle ground. For the threads that I want to see every reply I'll subscribe, for other threads I'll only follow up if I navigate back manually or see a quote alert and go back after that.
I would support it being optional, as well as having a forum-only alert option. The forum I use the most with an equivalent system has notifications up in the top-right by your user name, near where the CFC private message alert is. I find that works well - easy to check, but not in-my-face if I'm just visiting to check/update a particular thread. I probably would find it not worth it if there weren't a way to disable email alerts - in that case I'd probably have to switch my CFC account email back to a throwaway one, which would be unfortunate.