You don't have to be directly hit by the attack, you just need to see the enemy use it, and then defeat them... and hope RNG is kind to you.
Actually, We can't get the Egi until we defeat the corresponding Primal, so it's kinda there but not really.
I think SMN can benefit from BLU's spell book feature, but for SMN they can pick from a collection of Primals they've defeated as summons, which just sounds like a glorified Egi glamour system if it's Egi so I hope it's Demi summons because those are way cooler.
I think that would actually be a cool implementation. Personally I like the Demi-Primal idea more and hope they move in that direction. I like having the primal out for moments at a time to do what it came to do and bounce. The Egi's are fine and all but if we are going to keep them I feel like they need to feel more impactful. I do think having a cache of primals to choose from and assign (similar to egi glamour) for your Demi Rotation would be really cool. If they decide to keep both Egi and Demi then making lower Tier Primals (i.e. Shiva, Levi, Ramuh) for Egi's would be appropraite, and higher Tier Primals (i.e. Alexander, Bahamut, Odin) for Demi's would be appropriate. Even if they didn't change the moves or effectiveness of said primals for content, having the choice to customize your Job more would be GREATLY appreciated.