i would use em if they were just there anyway, though I dont often build them otherwise -- i'm not sure what the right requirement would be to have them auto-gen -- military base makes the most sense out of existing choices. Or maybe you could have it both mil base AND factory or something, if we wanted it to be thematic. I'm sure there are some balance things to work out though. One requirement i do feel pretty sure about though is that city should either be full HP, or at least in the green part of their health bar for any kind of auto-gen ability
maybe manufactory could also make 'em