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

[ViviAnimus]

Your first sentence does not make any sense. The comparisons are between top SMN DPS and BLM DPS per turn in Final Coil. In every turn, BLM wins over SMN, even though three out of 4 turns should be beneficial for SMN!
The best example is T11, which is a very movement heavy fight and SINGLE target. Yet BLM has around 40 DPS higher on average. Goodie, I wonder what's the matter. Probably those evil parsers.
All you say is speculation. You either come with proof that the parser does not reflect the true nature of SMN DPS and you show how it reflects SMN in a bad or good way (more damage or less).