Manipulation is the forte of every game, and every game developer. That's not a "good" answer, but it's the only one I have - and I've seen it in action in every game I've played since 1985.
Back to PnP games! Where players get to manipulate the DM.

That's the difference as far as the Greybeards and Blades. With the Greybeards, you never have to promise to keep Paarthurnax alive and after that he's tagged as essential or something. You're never bound to High Hrothgar to kneel in front of statues, in fact, they even say (and I'm paraphrasing here) "You've acquired some deadly weapons along your path, you can choose to be evil and feared, loved and kept in fond memory, or fade into obscurity". What you do from then on is up to you.
To "be a Blade" you MUST kill Paarthurnax to "uphold the Blade oath" (the same one that makes Odahviing exempt from death?) and be apart of their weird little ancient cult.
This is the kind of imbalance I saw as well. Seems like The Blades (Delphine, really) are deliberately written to be an undesirable choice and probably rejected. But then I was surprised by how many people got the Oblivion Walker achievement, considering that you have to become a cannibal to get it, so I am often wrong.
