Its not even that contagion would add up DPS, it just saves MP. T13 I'm ok on MP up to first Dive phase with garuda, usually there bard plays Ballad because nothing to attack anyway and then plays at the start of last phase so I just switch out ifrit while sitting on Neurolink because Mp wont be issue for the rest of the fight. First phases it will be issue, just because of multidotting. You can barely dps dummy for 3-4min, and p1+2 usually tends to take 5min ish so cutting out miasma2, even w contagion, and few ruin2's to save MP.

I'd use ifrit on every fight but so far its just UGH. T11 kinda need garuda on add phase, usually swiftcasting ifrit for the last add and last phase. T12 I'm dpsing bennus so the amount of running around with ifrit is just silly so I prefer garuda, who can start dps asap.