Hmm, the additional Potency of Thunder with Contagion is only 175 compared to 550 from the other 3 DOTS. It's nice to have fights where you use Ifrit over Garuda and vice versa, and perhaps even fights where you should switch pets for optimal DMG. But at the point where Ifrit can out DPS Garuda with Raging Strikes + Contagion on 3 DOTs (4 if you include Miasma II which is better than casting Ruin with Contagion boosting bonus Potency to 600) makes it very unlikely to find a spot for Garuda other than DOTs > Contagion > Bane.