Naturally, spoilers ahead. Though I figure most people would get that coming to this thread.

I think it makes perfect sense, honestly. As noted in the quest by Ifrit, we have the ability to summon the primals. I think this ties into whatever the Echo really is. If you remember, the Garleans have branded all those gifted with the Echo as allied with the Beastribes, and are to be put to death. The Garleans are also on a crusade to destroy the primals.

One can simply infer that the echo is the key to our ability to summon the primals. Now, the whole issue of the primals corrupting the minds of those that summon them is easily explained. When you do the Ifrit hard mode quest, you're told that you can only remain within the Bowl of Embers for 30 minutes before you risk corruption. That's the reasoning for the timer in lore, and after 30 minutes a magic spell that was placed on you removes you from the Bowl of Embers so you don't go mad.

With that knowledge, I think it's safe to assume that the Echo also boosts the resistance to this corruption. Keep in mind we don't summon the Primals for hours on end, it's a short duration. We'll summon them, they'll whoop some ass, and then vanish with the pact complete.

Such a short duration won't corrupt our characters, as they have that innate ability to resist it because of the Echo. The primals simply won't be around long enough to do any serious damage to our characters, just like in the Ifrit battle.