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

[Leonus]

I love this. The dps gap created between melee and ranged is just a complete design flaw with Coil's mechanics...

Coil as a whole? Or just final coil? people could have done COB and SCOB without a melee (or just one for a specific situstion). 99% of groups won't do FCOB without a caster, certain things are made easier by having them. Same could be said in reverse. I'm fairly certain, People Could Do FCOB with just all casters for dps too. If ranged dps could match melee dps output, most groups for raiding would opt to have no melee or just one for LB like a lot of groups did for COB. It wasn't until SCOB that some groups brought more than one Mdps and it started to become balanced between 2 rdps/1 mdps and 2 mdps/2 caster dps groups.