Making public groups opt-in is probably a good idea. However opt-in wouldn't change much for private groups because no one else can see the private groups that you have been added to.
Except, given Facebook's history, I'm assuming we'll have to visit each and every one of those groups separately to unjoin them. That certainly would change my relationship to Facebook as a service if there is increased overhead in cleaning up the effects of unwanted features that are imposed on me.
Making public groups opt-in is probably a good idea. However opt-in wouldn't change much for private groups because no one else can see the private groups that you have been added to.