I thought it was actually policy that as soon as a legion was raised, they were moved to the opposite corner of the empire... Quite a good way of avoiding having a large number of heavily armed locals with ideas above theirstation? I point to Legio II Augusta (barracked upon Caerleon in Wales) who had been raised in Iberia only a few years before the commencement of their active service in Britannia. And I think Legio XX Valeria Victrix came from Syria? Possibly? The point is, they certainly never went 'home' again...