You don't get better chances for a drop by killing higher level mobs, only for completing a higher level strike. If that was the case, everyone would be farming the mob spawns on bosses. The people that farm for engrams by killing enemies are playing the level 8 strike or are in patrol. The ones in Tiger Strike are farming the end roll. The only point of killing the starting enemies would be to have the slight chance of dropping an engram, but let's face it, do you really think killing an extra twenty mobs is going to drop a legendary?