I don't think helmets dropping are affected by whatever else is in Oryx's loot table. Seems to be a rare, random extra drop alongside primaries. The chance at everything else seems to be a separate drop. Regardless, helmets have always had a low drop rate. They are the most expensive vendor armour for a reason. I'm not sure what reason, but there most certainly is one.
And this is a problem. A loot game where end-game loot is so rare that people with even a dozen completions haven't even seen the armour piece drop (yeah, that's me)?
Bungie either needs to increase the drop rate of raid gear, in general or change the role that shards play. I've had so many people say that they could use it as a currency to buy high light gear from Eris/Vanguard, if only Bungie would implement it that way.
Is Bungie scared that people will leave the game if they have all the end-game loot? Some might, sure, but some are going to leave soon anyway, due to the huge amount of games that are about to be released. BUT, if they had been able to get all the raid gear easily and without all the fuckery involving raid drops(Example - I beat Oryx and all I got was 2 shards), they'd at least leave with fond memories of the game.