It start with a false postulate.
Melee classes does less damage than MRD : False! MRD are behind smn, brd, monk and dragoon in term of DPS. MRD have a great (very great) ability to deal burst damage, that why they are so popular (burst is a key mechanism in PvP). Monk are more utility and substainable damage oriented than dragoon when dragoon is more burst damage oriented.
The real problem about melee dps: most of them are bad/average and there is a huge difference between good melee and the average melee in terms of damages (because of the positioning as you said). At R30, I observed that some monk or dragoon always "outdpsed" my premade MRD when some did nothing. And more than players, It also really depend of how the fight goes (which one of your team will be focus/controll/kite).
Melee classes does less damage than summoners : True. Because the way of Summoners are (multi-target dps), they are the class that does the most dps in arena. Since 2.15, summoner have less utility than others dps (or are equal at least) and because they are caster, they loose dps when they are focus. And by the way, smn mostly disappear for the profit of Blm after 2.15. Blm does far less damages but are way more gamebreaking for the moment (you cry about smn a bit too late !).
There is not competitive 4v4 in FF (which make no reason to be optimized) but i'm pretty sure that MRD + SMN + Heal/Cac is not the best composition. PLD and BLM have more to offer in teamplay. IMO, if they let the limitation (1 melee/ 1 distant), DRG/BLM will be the best dps duo. If not It will be SMN/BLM, DRG/BLM, BRD/BLM.
2.25 will bring new rank, new gears. Mb a balance or news skills and we don't know how frontlines will works. But if it's "Bus vs Bus", we can guess Blm will shine followed by Smn. Tank will have to sponge damage/peel. And melee... will have to be opportunist.