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

[Dwill]

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.

If you use Energy Drain to begin with, it's a DPS loss. There is such a thing as a middle ground. Waiting to use Energy Drain when you are at 100 MP is pretty worthless but using it when you're still above 2500 MP isn't much better.