Slitherine

Is there any benefit to doing this? There seems to be negatives.
Thanks.

I think they help you fight other religions, with their own troops.

They work at triple strength to get rid of heretics. And usually if you are close to Pagan or Muslims, they will help.

Note also you can plant them in someone else territory!

Nothing quite like setting up a group of armed religious fanatics in the territory of someone you don't care much for.

Only heretics or all of other religions?

Heretics and Pagans, this is the (Teutonic Order) Way!

So no burning of muslems and ortodoks?

Nope.