Mr. Shadows
Nomad of the time streams
I usually only build encampment buildings if I'm playing a civ that has a unique, or as an after-thought/ just because. When that happens it's almost always a stable. Encampments are mostly useful for getting a GG in my games.
This is still true for the most part because if I'm going to war in the Ancient and Classical eras I'd rather spend the hammers on new units than the building. However, recently I've been waiting until the Medieval era to take advantage of Winged Hussars. That usually leaves time for a stable in my encampment. Winged Hussars are so strong that with the stable they get their first promotion with little or no rest. The insta-heal means they can keep moving and maximize the window in which they completely out-class the ai. Granted the GG is much more important but in that situation the stable pays off.