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.
One thing to note is you can force any female to be your concubine if you have them in prison. So, when you go raiding and capture a Catholic or Muslim then you have the option to get a concubine from a different religion. You, of course, also have the option to get other pagan concubines through asking or capture.
You can take female rulers as concubines and any child born to a concubine will have the same rights in succession as other children, but will be viewed poorly by others. This is a pretty sneaky way to get territory to fall under your domain. Just be ready to fight for it later on due to the opinion penalties.
Another benefit of stealing a ruler's wife is it denies them from using the wife's stats when considering state scores. So, if the wife is an Elusive Shadow and you snatch her away then the ruler is suddenly much worse at intrigue. Bonus points if she was set up as spymaster for the ruler.
If you make a married woman your concubine it will break her marriage. I'm sure you can think of all kinds of chaos this might cause. I doubt you would have much luck getting the now single ruler to marry someone you want but it could work.
It is a great way to change your religion. Once you have a concubine of a particular religion you should have an intrigue option to swap your religion to whatever the concubine's religion is. This is a very effective way to get out from the terrible Gavelkind laws of Paganism.
As you mentioned, it can get you more children which can be a blessing and a curse. This is mostly helpful when you start as an older ruler who is married to an elderly wife. Usually, you have a pretty terrible heir already in line. Grab a concubine and get you a better heir, just beware the murder plots from your wife and other children. Of course you may have to do some murdering on your current heir, but that's Crusader Kings for you.
If you force high ranking nobles to be concubines you can earn prestige (as mentioned in the Reddit thread)
Sources:
