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

[Aikaal]

There aren't really any uses for a second Super Virus and slow in really any encounter though.

Pretty sure that BLM gets a 40% slow too though.

Blizzard 1 gives 40% movement reduction. If you want to go further in this argument. Black Mages are superior because they also have Letargy that is a 20% movement reduction + 20% slow on skills and spells.