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

[Cebo]

This is a lie. Let's look at those utility spells.

• Super Virus is reduced to a regular Virus because you have a Scholar. So both BLM and SMN Virus is negated.
• Eye for an Eye can be used by BLM, they are negated.
• Combat raise is not viable. You need mana and healers can handle 2 to 3 raises easily. In farm content, you will never combat raise because your party won't die to farm content. The only unique utility spell becomes void.

You missed my point, since it appears we'd be in agreement. I'm only saying SMN utility is better than BLM's in a vacuum, if you ignore the fact that other classes can provide that same utility, notably scholars. Since SMN utility is already covered by other classes, they don't bring anything of value to the raid.