On our 319/320 the cockpit EVAC command toggle is permanently wired to the CAPT & PURS position. The forward and aft FA stations have the ability to initiate an evacuation under specific circumstances. (Bent/scraping metal - with an inability to communicate with the cockpit).
Considering the first people to an airplane accident are the pilots, and that those pilots may be the first casualties of the accident, I think it prudent to train FA's that in very specific and limited circumstances they may command an evacuation.