Like some "pro" players believe must be done. And let me share with you the results I've seen from some of the same "pro" players who have chastised me for NOT defending Nis, instead making a better and more effective stack that defends further south.
I spectated one game with one of these "pro" players on Serbia. He put his stack out in Nis. He was playing against a pro AH. AH started by cutting his stack off from reinforcements, capping South Serbia, completely isolating Serb general, and then vaporizing what existed of the Serb stack on about turn 8. By turn 10, Serb was about totally dead.
When playing against a pro AH, there are two possibilities here. Serb either manages to get such a wonderful stack in Nis that AH goes right around it, walling it in, isolating it, rendering it useless for anything but defending Nis, quickly overrunning Serbia. Or, AH quickly vaporizes Serb stack in Nis and overruns Serbia.