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

[point09micron]

These jobs are support jobs but BLM and SMN are the ony ones that does not support damage.

BLM
- Virus reduce enemy damage
- eye reduce enemy damage
- Apocatastasis reduce enemy damage

You can't call BLM support when 2 of the 3 abilities you list are cross-class from Arcanist. A more reasonable statement would be Apocatastasis, Lethargy, and Sleep. Two of which don't work on most 8-man content (Lethargy works on Scylla and I think Glasya, but that's all that I know of).