I got 2 in about 15 minutes grinding mobs in cosmodrome- first from a random blade that showed up, and the second from another blade that showed up during the big Fallen-Hive battle outside the Terrestrial Complex. Didn't even notice I'd gotten the second one in all the excitement. Haven't seen another one since then, and one's on its way to being a well-leveled Eidolon Ally. So, I doubt it's anything other than rng, but I'd be curious if having one does alter the numbers or their randomness.