10% is alright you say? Let us see. BLM 550 DPS - SMN 500 DPS. 10 minute fight. Boss HP = 1 million
SMN total damage = 300000 | BLM total damage = 330000 damage. So BLM does 3% extra damage in terms of 100% boss HP. Also, in theory, BLM does around 12% more damage. So it's probably around 4% extra damage on a 100% scale.
Considering this, BLM does a lot more damage, has the best single target damage, has the best AoE damage and has the best utility at this point in time. So yes, it does matter that SMN is outshined in every aspect of the game because it makes the class very unattractive to play and keeps players like me forced to play BLM so get the best out of it. And I am a pretty good Black Mage. I want to be able to play Summoner again and pull those numbers, I would actually like higher numbers, because let's face it. Summoners are worth doing at least more ST damage.