I was just about to post about this (just noticed it today) but did my due diligence and searched first.
I definitely agree that it's less likely this is deliberate considering how crazy rare these items are (0.08%).
For just one spell, you need to probably kill 2,500 zerkers. For our raid, we want 14 spells (out of 18).
That's 35,000 kills!
No one would choose to do that (indeed, the last time we did the zone, we only got a couple spells before we tired of the grind and gave up, having finished everything else in the tier, not just the zone).
The alternative is soldiers (much more reasonable at 1,220 kills but still way over all other requirements in the zone).
All the other drops are the same except these two with a probable kill count of 700 or so to equip 14 toons with just one of the four items required for a spell, which is much closer to reasonable (Blood Raven is 1000 kills total).
Unless the intention was to make these spells some of the rarest items in the game (where you're punished arbitrarily for making the mistake of choosing zerkers instead of soldiers, because you didn't research alla), I really do think this was much more likely an oversight than deliberate
8% a drop, considering you need 4 of each per spell (and four different types of each) feels about right to us.
I'm happy to list the npc and item ids/links if that would help, if you'd please consider changing this. Appreciate totally that you're very busy and probably already have lots on. We'll switch to killing soldiers in the meanwhile.