Odd, in my games I tend to see the exact opposite. In a recent conquest of Burgundy a rebel stack spawned in Vlaanderen, one that would defect to Netherlands. Great! Weaken Burgundy even more, after I peace out, and Burgundy is without any army, this 15 stack rebel group will knock another province or two off them. Nope, my ally Austria broke off a siege (though one that was only at -45%) in nearby Hainaut to go attack the rebels. It's precisely the kind of scenario where I would ignore a rebel stack. One of my favorite things is to completely destroy a countries army, and once a few rebel stacks spawn, take the peace and let rebels continue the destruction of a country while Burgundy/ Sweden/ etc. has no forces to deal with the issue.