90% if the game runs on Energy...
Going to dungeons requires energy. (Minor, if this was the only use for energy...)
Crafting items cost energy. (Critical, every since the patch crafting never seemed less worth it)
Reviving costs energy. (Major, in a single T3 run who knows how many hundreds you might spend)
Upgrading slots cost energy. (Critical, if you want to keep your slots you have to pay 100-700CE a month)
Opening danger rooms and treasure rooms cost energy. (Minor, Danger Rooms are worth it, treasure rooms aren't)
Energizing a mecha knight costs energy. (Minor, but I'd take a chance on mecha knight vial)
Trading items sometimes cost energy. (Major, in my opinion we shouldn't be able to trade CE)
Edit: Of course I know you don't always need to spend energy on reviving, just basing the facts that it still is an option of doing so in desperate measures.
To correct you
Reviving=doesn't ALWAYS cost energy
Energizing mecha knights=a waste of energy
Yup CE is far far too valuable and the death of the game is coming.