Maybe it was forbidden because people got sick of seeing Titan in a thong.Summoners description is "Mages that have unlocked the secrets of the forbidden magic of Vana'diel". I mean wouldn't forbidden magic mean its forbidden for being to devastating or overwhelming ?
Summoning was forbidden before Karaha-Baruha resurrected the art.Firstly, Summoning magic is forbidden not because of its strength but because the Windurstian hero sacrificed himself to summon Fenrir.
I really hope you mean mythic because lolClaustrumlolmy friend has everything you could possibly get for SMN including his relic.
As others have stated SMN isn't a purely DD job so of course it's not going to out damage pure DD jobs like BLM & SAM -unless there's a big gear/ merit/ etc. gap- but in abyssea it can get almost on par.
As much as I'd love to be able to fire out BP every 20-30 seconds being able to do that while also buffing/ curing/ enfeebling in abyssia hate free would be broke & doing it outside abyssea would just burn your MP to the ground.
I seriously doubt BLM, SAM, WAR etc. could constantly be spamming such high damage as has been suggested without dying (BLM especially) unless they have an amazing tank or your talking about fighting mobs that die very quickly as I often pull hate on my BLM if I cast to quickly & I only recently returned to the game so I'm missing a lot of atmas gear & elemental magic skill so my nukes do less than 5-7k damage.
With the abundance of 2hr chests you get in abyssea SMN can also go off & insta-kill mobs on their own to. If you include 2-4 mobs entire HP pool every few pulls you probably are out damaging other DDs. I used to love doing this with Odin but I'm told with the new MAB & element attack+ atmas a SMN & a sleeper -not sure if ones needed I haven't tried this myself yet- this can be done to much greater effect with larger groups of mobs & normal avatars AFs.
Over all with the incredible boost SMN got in abyssea -and unfortunately who cares about anything else at the moment?- I don't know what your complaining about.