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

[Orrias]

Only a small percentage of players like this current iteration of Summoner.

Which is based on what -- anecdotal evidence..?

Most players quit playing Summoner and moved to other jobs because their more satisfying to play while the Summoner has a boring play style and is hard to play there is just no fun factor in playing Summoner vs Black Mage or White Mage.

If they, (whomever "they" are) did in-fact switch to another Job, then they'd encounter a myriad of issues; latency (i.e. mudras), getting to grips with the new Job(s) tempo of play/ rotation, as well as (possibly) having to level the Class and/or Job up to 50, plus gear it.

Minimizing gearing/ learning difficulties, most of these people you refer to, would most likely have swtiched to BLM, however, the BLM rotation is dull, and the most advanced play revolves around the server ticks of mana and pre-positioning.

Looking at WoW if the Summoner was slot in Crowd Control and enfeebling to increase the DPS of the party maybe it would be good but Summoner just has no place in this game now. If SE based the Summoner off the Elementslist/Shaman and made a better pet system the Summoner would've been great.

"Summoner has no place in this game now"...

Also, SMN is more comparable to WoW's Lock, minus the Drain abilities, which could be something to look at.

Regarding Shammys, they're too much like a nature version of a quasi Red Mage, and the classes mechanics are just too off to fit in with the FF series' Summoner class/job. From my recollection, Shammys had a mana restoring totem, in the past. Having/ using the Egi as an aether/ MP battery is an option. In additon to this, Shammys also have a more elaborate/ advanced version of Aetherflow (i.e. their charge system). I could see a system whereby SCH's keep Aetherflow and SMN's get a modified version of this to help with greater mp returns/ regen, and possibly additional benefits too.