I've always liked the idea of having two series of tanks, basically 80t ICE and 80t SFE that took the place of the Demolisher, Screk, Partisan, and Rhino/SturmFeur. ICE is the cheaper unit, with the SFE being used to outfit the best units, especially in the Succession Wars era.
You also have to remember when designs were originally built/designed. 4 shots with ACs on the original Partisan had a better change at critting a fighter/VTOL then 2 shots with PPCs, even without flak rounds. ACs were also considered easier to build/repair/maintain vs an energy weapon. Remember, places like the FWL had trouble building PPCs fluff wise.
The Zhurkov SRM launcher makes sense as a crit seeker after the AC makes big holes.