It's a design flaw really. They tried to make Ifrit and Garuda do the same things and it failed. I've made post about this and would have much rather had one pet for AoE, one for Single Target, and one for Tanking.
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I don't have the exact numbers up right now, but the mana saved by Contagion is under 50 a minute. If you use Miasma 2 with it, Contagion is an increase in mana spent per minute by nearly a hundred. I thiiiink the actual number saved per minute by contagion was only around 19, but I'll bring the actual math back up later if there's interest.
The gist of it is that you save less than 1 cast of the dot worth of mana, since its less than a 100% increase in its duration, and you also have to subtract whatever mana you'd spend on ruin instead from your savings.