Yes it's nice and cosy then isn't it.
With a nice little message saying you and your allies have conquered the province and because you have the larger force the province gets handed over to you.
Another nice little wrought..

In this situation I prefer if the rebels stay and fight.
I like actually going to battle in this situation, sit back let my 'ally' take the brunt of the damage, maybe join in with a few archers where stray arrows take out both troops and then still get the province because of superior numbers but in this situation your ally will lose a lot more numbers than if it were auto-battled