This is a really great point. I'm unwilling to try to determine if the distribution of items that are dropped from each quest are evenly distributed, but there's a bunch of anecdotal evidence that it isn't. It seems like half the people on the forum have had common experiences of getting dozens of unwanted items before getting the desired one from a quest. (Lots of people complaining about having to ransack House of Pain many times for the Family Recruit Sigil, for example)Here's the thing, even if the percentage is 33% on elite, which I don't believe for a second, that 33% is shared with all the named items available for that quest. So Myth Drannor for example, there are 2 variants of every weapon equaling 100, there's the 3-5 named items people want, and all the solar/lunar gems which are about 60ish (estimate) This adds up to be approximately 165. Now you have to divide that .33 (33%) by 165 and that's closer to .002 or .2% for an item you want. That's 2 out of 1000, or 1 out of 500 if you want to reduce it to simplest form. And have you ever opened a chest, got a named item you didn't want, reroll and got no named item? Yeah, that happens more than you'd think. How bout once you get a named item it locks you into a named item category so that next reroll guarantees you a named item.
I personally don't really want them to add more incentives for people to spend money on shards because I think getting "base" named items without mythic or reaper bonuses should be easy. I don't think you should have to ransack a quest more than twice to get everything it offers. That's already 16 runs in short succession of exactly the same content - I don't even want to do that, I'd prefer to run a diverse set of quests.
The rare items are just garbage, I have no interest in that at all and will refuse to spend any money on the game until they reverse that in existing content and stop implementing new ones.