While I normally use Garuda, mostly because i like the caster type pets, I personally don't care about contagion all that much. While MP is important, I don't think a Summoner needs to watch it as much as the other magic classes. Normally when i use contagin I use it after I used Raging Strikes to increase my attack power, that way the added 15seconds are more beneficial, this is mosty on bosses. For trash I usually use it and then use bane to spread the extended dots to all the enemies.

Also, since no one probably used it after they got summoner, did anyone notice that Emerald Carbuncle has contagion too, lol, different name but same move.