Aside from giving Summoner itself Contagion to at least make Ifrit more worthwhile to use, they could always give Ifrit an ability that increases the potency of the master's DoTs on the enemy, balanced to do around the same damage as the extended DoTs from Contagion. It'd provide the choice of wanting to have more "bursty" DoTs with the catch that you'd expend more MP in a given timeframe due to reapplying DoTs more often over longer DoTs that conserve MP and can be used to give more leeway for using Bane.