So long as it fulfills the strategic plan, a rebel province is obviously just itching for some good governance from my munificent and glorious empire "Thou shalt not suffer a rebel to live" and all that.....

In my analysis rebel armies fall into two categories: 1: easily whupped, therefore not much of a barrier to my enemies; or 2: headed by a good general, but being a rebel he is (relatively) easily bribed and therefore would be a threat in the hands of the enemy. Either way, I don't hold with the "buffer zone" theory, but as I usually play in Domination mode rather than GA that may colour my analysis - the only reasons I apply for NOT conquering a province are 1: it would be too vulnerable or weaken my border armies, or 2: I don't want to go to war with its current owners (yet).

The other big threat of creating/maintaining rebel lands is the dreaded re-emergence. A province that one year has a paltry handful of grey peasants may the next harbour ten stacks of a once-vanquished enemy. Not great on your border, as this cash-strapped re-emerging faction will be looking to expand!

Contrary to Sensei Warrior's experience, I HAVE encountered rebels attacking. It's not common, but also it's not impossible. It seems more likely to occur where a rebel faction already controls more than one province. It's not wise to turn your back on rebels any more than a dodgy neighbour.