You'd generally want redundancy, but anti-air guns (AAG) are useful for very short-range and low-altitude engagement; they can act as quasi-CIWS, help against targets with self-protective suites (that might jam radars and/or scuttle infrared), etc. Modern AAG such as the Korkut can be tied to one's radar, which can automatically direct the AAG to those targets. Pair these AAG with the command-line-of-sight HQ-7s and/or short-to-medium-range terminal-homing SAM - e.g. Hisar or Umkhonto - and you have a solid low-altitude air defence layer.