Plus the guns are useful as a secondary 'self protection' mode against light armour or recon vehicles... (ultimately they can also be used deliberately in eg urban environments but this is *probably* a waste of limited availability assets).

A SAM (9K31, 9K35) would only have self-protection from crew small arms and (possibly) one RPG22 per vehicle.
Tunguska or Shilka is a credible threat to an M2 or BMP, or M113 or BTR/BRDM class vehicle and obviously also 'jeep' or HMMWV... although fire and retreat would seem preferable to hanging around to find out what else is there....