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

[Romsca]

BLM has slightly better utility because it's actually useful as a BLM. Raise can be alright, but no one brings a SMN for their raise, nor do the majorty of SMN have to use Raise. It does have enhanced E4E and Virus, but those are again, very situational skills in which the SMN does not use it most of the time. Especially in Final Coil, I have always felt that having manawall, manaward, apocatastesis are nice bonusses to what a SMN can't do or actually should do in order to make it have better utility than Black Mage.
I would argue that having no unique utility, at all, makes the utility you have weak. Gigaflare? SCH should use the virus. Nerf cloud? SCH should use the virus. E4E? Useless most of the time and SCH can use it. Raise? Total MP killer and a SCH with good MP management will raise over you all the time. SMN has the potentional for good utility but doesn't have to use it at all. This can be said about BLM, you don't have to use those three skills I mentioned, but they are very handy because they are unique. Manawall during earth shakers or to take the Benu? Takes a lot of stress from the healers. Manaward? Saves my ass a couple of times. Apoc? Godly on Akh Morn, switching of the flame, weakened target for Imdugud Charge, Weakend target or target with stacks during nerf cloud ... BLM to me at least, has been a LOT more useful in terms of utility.
Although I would say that well, having that Raise can be very handy or when SCH is dead, it can be handy to use virus yourself. I would however, from my own raiding experience, still claim that BLM has better utility, but not superior.

I somewhat agree that summoners utility outside of dps is lacking. I wouldn't over look the value of two supervirus in the group. And If black mage had access to resurrection, not raise from cnj, then black mage would probably be expected as the go to revive due to their high combat mana regeneration. Now if summoners had a much larger mana pool, which is what we should have due to aetherflow mechanics, then they would be first for revives unless swiftcast is on cd. But this just a hyper theoretical look at the situation.