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

[ViviAnimus]

I test again 7 min 9 sec ifrit did 176 dps caster 341 dps. 517 overall dps. I can save lot mana with very minimal dps loss and I have more mana in party. If i can do max potential dps with auto attack and with spamming ruin2 that not mean i should do it in that way and i did not even do that in my 3 min test.

Please watch the video I posted and tell me where I am going wrong. I am almost on 450 DPS with a basically perfect parse and I was OoM after three minutes.