Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Sunako]

Energy Drain should also never be casted during the initial Raging Strikes, it just result in a damage loss due to having one less Fester and early on, the additional MP will just be overkill. Even later on during an encounter, Energy Drain should be used when you're starting to get low, not when you are in Raging Strikes.

I said in case you need mana. If you need use that energy drain later without raging strikes isnt it dps loss? If you know exactly how many energy drain you need in the current match, you should use those during cooldowns are up and not at the moment you have 100 mana.