I just discovered in a recent scenario of mine that this happens with other units as well. I had given the barbs three cities and each city had one of the three Dragons in it. They were all disbanded as was an Ogre Warchief and a PitBeast. In their place the AI built standard Axemen and Archers. Hoping to avoid this I had placed Markets and Courthouses in the cities and also placed a couple Towns. But rather than it simply being a unit cost problem I wonder if its something else since the AI replaced those units with other units. Kael addresses the disappearing Archeron in Shadow. I hope this fix addresses the value the AI places on its units. I've noticed the Avatars army is also culled in a poor way. And I've had AI civs disband powerful units that I gifted them via the WB, yet they had hordes of weenies.