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

[ShinryuReishiki]

I doubt it. Every point of Piety gives every class/job the same amount of MP (8 at level 50), they won't adjust it just for one job. Or at least I highly doubt they will.

Each class actually gets unique amounts of MP per Piety. SMN gets ~7.25, SCH ~7.5, WHM ~7.9, BLM ~8.25, and PLD ~3.9. These aren't exact numbers; I just tossed on some Piety pieces, found the difference in MP, then divided by the amount of Piety. A SCH with no Piety equipped actually has the same base amount as a BLM, but is a few hundred MP lower...