1. The problem with energy and energy potions is that the way they are currently working is that energy potions completely cancel energy out. You can carry as many potions as you like. If you're low on energy, you hit the 'e' button, which will restore all your energy. So, you'll never run out of energy. Thus, the unlimited amount of energy potions as well as how often they can be used and how much energy they restore make energy a completely unnecessary component in this game.
2. Energy has become a useless annoying gameplay element. As a result, every skill and equipment piece that gives bonus or penalizes energy is nearly useless. Also, energy could be an added tactic factor to the game
if it was set up properly.
EDIT: I just reread the OP and the reason for nerfing energy potions is not stated at all, sorry for that

Another reason for 2. : Masteries that give energy instead of health are basically screwed. They get an almost worthless attribute...