I should make clear that I only supported the rebels after they revolted so I could tell if they had a chance of success (two 18-21 stacks compared to Sweden's 18). At that point, I only did it to keep them from sieging my territory and so that Finland would be friendly to me instead of wanting to throw me in a ditch.
I also had the advantage that I could monitor the rebellion as it was a border region...
There are times when it's worth it and times when it's not. I wouldn't do the same for Ruthenian rebels in Lithuania or Breton rebels in France.
However, this has basically allowed me to dominate both Sweden and Norway easily, so it's a strategic boon for me as Russia. I'm debating if I even will take Crimea now...
I tried that. Maybe it's a recent patch change but I have already rebelling rebels greyed out for me when I try.
I also spent a while trying to support rebels for a Hindustan game I was trying to play right after someone took a bunch of territory which should mean overextension and nationalism for easy revolt but nothing happened.