Yeah the encampment buildings are pretty low on my priority list too. If I end up needing an encampment for the strategic resources, I may just end up building one for the great general points, but I'll rarely produce the barracks before the units I need. One exception is if I'm intending to start my warmongering in medieval. That's late enough to consider building a barracks before starting on my troops. The production bonus at least feels like it somewhat makes up for the cost of the building. It's not high enough for me to build it just for that, since it takes as many as 80 turns to be paid back, but at least it's not a great loss production wise if you happen to not really know what to build at some point.