I'll take a stab at this as best I can.

I do not really go out of my way to make rebel lands, but I'll tell you the reasons why I capture any provence, rebel or not. I'm pretty sure that my reasoning will be universal so it will fit for deciding on capturing a rebel provence.

1. The provence has strategic merit. It sits at a natural chokepoint, or is easily defended. It might shorten up my "exposed" provences, meaning the ones that border another faction. If that provence is controlled by rebels then I probably won't take the provence. I have never encountered rebels attacking another provence, meaning the rebels in the provence of Wales won't attack Wessex. In this case the rebels gives the provence more strategic merit, another faction will have to defeat the rebel army to occupy the provence therefore the provence is harder to gain. This seems to be the reason why you make buffer provences.

2. The provence has economic merit. It has trading goods, or good farm lands, aka its a money maker. I will also take it if when the appropriate buildings are built it will significantly reduce the maintenance costs of my fleets. In this case, I will take the provence when it is convienant for me. If the provence is controlled by rebels I will try to purchase them, thus giving me an automatic garrison. If that's not feasible then conquering is acceptable.

3. The provence has tactical merit. It has a valor bonus for units I use like Chiv Knights in Toulouse, units that are available no where else like Galloglasses in Ireland, or the provence has IRON. Depending on various factors, like how much I use the unit or how good the unit, will dictate how quickly I will gain the provence rebel or not. A provence with Iron will be assimilated as quickly as possible.

There are some minor considerations as well. In a GA game I try to keep a moderate size kingdom. I will not go on a conquering spree so I won't take amy provence unless it will clearly be an asset.

In a world domination game one thing might save the rebels. If I remember correctly rebels will trade with you if they have the appropriate buildings. In that case, if there is a provence with the appropriate buildings controlled by the rebels I would go out of my way to keep them rebel so I have someone to trade with in the late stages of the game, when money gained from trading is shrinking. I say would, because I just thought of it, so I haven't tried to see if it would work.

I hope I answered your question, and I will apologize if the post was a little to lengthy.