Anon,
if you let them settle down, they get to pump out lots of free upkeep units. on the other hand, this dilutes their veteran stack/high dread/high command generals.

i took them out before they settled down in turkey and egypt campaigns, but in my russia game they have already wiped out egypt, made turkey a vassal and are now smashing up the byzantines. i'm attempting not to fight them now so that i'm not in the middle of an expensive, offensive war when the timurids show up...