If migrating I always keep the settlements for as long as I can. Although I migrate everything useful from them, I always maintain one unit of peasants to hold out in a seige should one arise and keep the populace at bay.

Regardless, I don't develop these settlements, and simply leave them for enemies to find and take off my hands. Even then I hold out as long as I can in the case of a siege, wiping out all my buildings whenever I predict an assault is coming.