Red Mages have less mp regen, But their skills cost way less mana. Red Mage's most expensive skill is 600 mana. A Summoner's dot cost 720 Mana. Using Aetherlfow only gives you enough Mana to use Ruin 3 out of dreadwyrm trance once.
Considering that Summoner wants to ruin 3 as much as possible (in dreadwyrm trance and out) Cause it's a dps gain - 150 potency vs 100. only 30% more potency on a class that is severely lacking damage in the first place.