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

[Romsca]

I do grasp the design concept, I said I do not like blm' playstyle, and therefore not an authority on it. Instead of your endgame rotation being around your top spell, Square Enix went with you casting your weakest with hopes of proc'ing a bonus to freely cast your biggest attack. This makes room for an erratic and wide range in spike damage. I am not calling for a blm nerf, but I think blizzard 1/3 spell potency and firestarter proc chance should be lowered.