In my current campaign as the English in 1124 (Expert, vanilla MTW), I have raided some German and Italian provinces, with rebels sometimes moving in. Sometimes I prefer to leave these as rebel lands which can provide buffer zones to enemies and slow neighbors from coming as they need to attack the rebels to get access to me. I was wondering when players decide to grab the rebel lands, and when do you let them be?

In my case some of the rebel lands I couldn't hold and didn't want to as they are non-productive lands (no resources, not great farm lands). I am considering grabbing one for a trade item and some income, though I am also considering letting it lie, though it would be another coastal area, (provance I believe), so that may have some advantage as a place to build ships.

When do you create rebel lands as buffer zones or for other reasons, perhaps as training grounds?