Just thought to add as it relates to the discussion...

I am an XJT CA and I will put just about any pilot on the aft jumpseat for whatever reason (W&B, jumpseat MELd, two guys trying to go, etc.) and most of our FAs are fine with it. I will usually give my FA some discretion when it comes to putting an FA on the aft jumpseat and it seems to be about 70/30 with them (in favor of granting the jumpseat). Just my way of avoiding a cat fight at FL370 where I can't watch!

That said, I can only think of one time where I overrode the working FA and put the jumpseating FA on an oversold flight (trying to get home for some emergency). Sadly, no catfight.