When playing as a Pagan ruler with the Old Gods expansion in Crusader Kings 2, one of the options for female prisoners is to take them as a concubine. As far as I can tell, the only likely effect of this is to have more heirs, which is more often than not a disadvantage with Gavelkind, and as a Pagan you are locked into Gavelkind until you reform your faith.
Are there any benefits to taking a concubine then? Either before or after reforming your faith.