(Sorry about double posting, I don't have edit privileges.)

For readers who have never tried mass migration to the other end of the map: a big worry that kept me from trying it earlier was that everyone around me would have built up their cities and I wouldn't have an infrastructure in place, leaving me vulnerable and underbuilt in a hostile land. This is not so much a problem as one would suppose; the whole idea is to take other people's infrastructures. You have way more soldiers free for campaign season; take settlements, merge, and move. You're conquering their territories, and they can't fight back by conquering yours -- they're too far away, if you still have any. Their soldiers are tied up defending settlements; yours aren't. When you take their settlements, you get the buildings they paid for at little or no cost to yourself. You have an overwhelming offensive advantage until precisely the time when they're so weak they can't fight back.

Advantages to migration:

1) It's just fun. History skips a beat, Fate does a double take, and your neighbors are very startled to make your acquaintance.
2) There's a reason to blitz your neighbors that doesn't have anything to do with powergaming. If you don't, you're dead. Get creative!
3) It can be very challenging. You're in a foreign land with next to no resources aside from your army, hedged about by people who don't share your religion and lagging a dozen turns behind everyone else. Survival requires that you make enemies and take risks. The resultant handicaps make the game much more challenging, and have far-reaching consequences.
4) You get to exploit (or be exploited by) the doctrine of asymmetrical warfare. Neighboring factions have similar styles of fighting. Changing regions will require that you adapt old styles to new circumstances, faster and better than your opponent as they do the same. You may have worked out how to get the most mileage out of Vardariotai fighting the Turks, but how are you going to handle those English longbows?