Disregarding aimed eye/head shots against helmets, other body parts are covered by either combat armour mark 1 or mark 2, both of which have lowered resistances to electrical damage. While the difference in DR/DT for both electrical and plasma are almost minute, the resulting damage will of course be substantial - numbers are everything in reloaded.
All I am saying is that there will be benefits for placing the +5 crit roll onto the plasma pistol extended cap, and no other weapon besides the plasma rifle will need this bonus!