Contagion is way too overrated!

[ShinryuReishiki]

There's no reason to make Ifrit run around. Stick it on the biggest, baddest enemy and leave it there. You don't need it to go chasing birds.

now when you factor in ifrit + melee buffs etc he will do more.

Even without slashing and blunt debuffs, Ifrit still comes out well ahead.

As for Contagion, people really don't seem to think about how strong it is with buffs.

No, it's definitely been considered. Without buffs, Contagion's DPS would be completely insignificant. The problem is, buffs are only up so often. In a 10 minute fight, Contagion would give you 60s of 20% buffed DoTs that you would not have had without it, but Ifrit would give you 10 minutes of 20% stronger pet DPS.