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

[Natsuno]

For average players the DoT system causes a lost of 30-60s dps because their damage ticks as one massive DoT on a global timer instead of each DoT having it's own damage tick and timer. Also Summoners initial damage starts of high but quickly drops off after 4min because of potency issues and how SE designed the DoT system. The Summoner is in the Saboteur slot which involves weakening enemies to increase party DPS but they are treated as pure DPS. These issues all combine to make the Summoner a sub par DPS.

SMN:
Healing received reduction
Heavy
Primary stat reduction
Blind
Slow
Bind


SCH:
Healing received
heavy
Primary stat reduction
Blind
Slow
Bind (x-class)

BLM
Heavy
Sleep
Bind
Slow
Stat reduction (x-class)

NIN
Stun
Silence
Slashing resistance reduction
Healing received reduction
Increased Damage taken
Heavy
Bind
Slow (X-class)

If anything, there's your Sabatuer job right there. Even if you discount stun and silence, it's still a tie with the fact that slashing resist doesn't suffer from DR in its favour.

edit: apologies if I missed anything. It's very late & I'm tired.