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

[Viridiana]

Some attribute the damage per potency differing due to physical and magical defense of targets. Personally, I suspect that Ifrit's physical attacks use the Physical Damage stat of the weapon, such as Dreadwyrm Grimoire having 82 MD and 58 PD (Although it's not shown in game), but I haven't done any significant testing to support such a claim.

The latter explanation seems much more likely, unless someone wants to explain why ladybugs outside Ul'dah should have the same defense stats as Bahamut.