The issue in general with having xp gained from individual actions, is that you end up gaming the system by maximizing some particular actions being taken.
That is to say, the equivalent of leaving an enemy mech up with one ML, intentionally showing an armored side for the ML to shoot at, rinse and repeat until all armor is gone, and then finally core out the target to finish the mission.
Or firing MLs at max range with a spotter, where your chance to hit is abysmal, but you're still generating 'spotting' xp for the spotter.
We're supposed to be streamlining from the tabletop, not adding more ways for the players to waste their time. This was done by equalizing mechwarrior xp, so you don't have to stall on taking a mission because you have extremely unbalanced pilots. Also reduces the amount of pilot micromanagement you're doing by having pilots flip mechs depending on who needed more xp when.
In a more philosophical sense: what is determined to be more 'valuable' for 'experience', and why? Usually get into chicken and egg arguments over what actions ought to be more valuable than what, because very little can be achieved in a vacuum in this game. No LoS = no indirect fire, lack of weapons = lack of dead enemy mechs, and so on and so forth.