Interesting tactic I have been using in my current GA campaign. I have been razing some provinces I took earlier (I didn't add to the buildings there, just what I 'inherited' ) to pull back my armies. By doing this I was able to free up some troops as I have a smaller front to defend, as well as allowing the Spanish to spread out and engage the Golden Horde (seems like their crusade is not fareing so well).
But I also poisoned the provinces I left by taking down the guard posts and posting my spies there (I have some also harassing the Argonese in Aquitaine and HRE in Provance). So in this way the provinces they take will be troublesome at best, as they have to rebuild and survive lowered loyalty while they occupy and put down the locals.

But of course, it is easier to use this tactic if you are playing GA as you are not as concerned with how many territories you take. Though it is possible to use this tactic if you're playing a conquest campaign to drain enemy resources (need more troops to hold down unruly locals) and spread the enemy out thin as they acquire territory you don't wish to hold or don't think is worth the trouble of holding.

Also my idea has been to allow the Spanish and Argonese to engage (as well as the aforementioned Spanish and Golden Horde) I am again attacking the Pope, allowing the Byz back in toward Rome and the Papal lands (after the Sicilians went bye-bye, not my doing, really).

Do you open channels for your neighbors to attack each other? How often do you do it in a campaign on average? Do you sometimes also mine the areas with spies?