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

[ViviAnimus]

Vines were very easy to deal with as a BLM as well and simply put, BLM does more damage on T6. You keep coming back to older coil where SMN pretty much got outclassed one very turn after the BLM buff. T9 and T7 are debatable. could easily push 400 DPS there while SMN was around 380 in ilvl 107 gear. I've never seen a SMN of saw a SMN outDPS me in sells either. But it was pretty sweet in that place.

In secondary coil, Manaward was useful to reduce damge on T7, extremely useful on T8 for the Allagan field and also useful on T9 for megaflare or meteors. It's negated damage.
Manawall was indeed pretty useless in second coil, but on fights like Titan, on Twintania and perhaps more, it was great.
What utility did Summoner bring then in other coil? Nothing much. The raise was kind of an option, but that's it. (CON)