using rebel lands as buffer zones

[Alexios]

I have found that rebel-held provinces can and will attack you on occasion. This happened to me in my current campaign as the English. I had led a successful attack on the Argonese in Argon, but lost it in the very next turn. A bordering rebel province (once controlled by the Spanish) launched an attack with two full stacks! It surprised the heck out of me, because I never knew this could happen before. But in the two years that I have been playing MTW, this is the only time I have ever witnessed it. So I think the odds are remote enough that I would feel secure about leaving my back undefended against rebels.