1. "Serve the Dragonborn" is BS. They don't KNOW YOU and what are your TRUE intentions. Claiming that they should serve and die for an unknown person they've just met is stupid.
Bull [censored]. After you "prove yourself on mission going to get info that doesn't exist, Delphine
refers to you as the Dragonborn every. chance. she. gets. They do to know me, and frankly these guys are about as helpful as a bullet wound.
Not to mention:
And by the same token, I don't know them or their true intentions. I do know that one of them deliberately interfered with something I was trying to accomplish in order to force a meeting with her. Then I had to prove I was valuable enough to be of any use to her, then I had to go on a wild goose chase for info that didn't exist, then I had to go find a guy who was a complete stranger to me and prove I was trustworthy enough for him to accept my help so I could reunite them. Then I had to help them find a safe haven and source of even more info and use my own blood to get it open because without that they were locked out.
All of which I had to do for a couple of unkonwn persons I'd just met, and who I was forced to meet because one of them used an underhanded trick to arrange it. When do they start proving to me that they're worth my time and cooperation and allegiance over the longterm?
Yes they were useful up to a point, but since they only seem interested in me as long as I'm useful to them and willing to comply with their wishes, I see no reason not to return the favor with equal gusto.
^ This applies in everything in regards to the Blades.
2. Killing Paarthurnax is a test to see if you want to become a part of their mission, if you wanna be a Blade and hunt Dragons. Being called a Dragonborn does not necessarily mean you want to dedicate your entire live to be one. Also you are the only one who can meet Greybeard's Master freely. How would you imagine a group of Blades entering High Hrothgar?
By this logic, since it doesn't matter that Parthurnaax has changed his ways, he still has to die for what he did in the past. Ok, cool. Going by Lore, the Blades have committed some of the
worst atrocities against every race in Tamriel. So, how about we remove those "essential" tags and let me punish them for their organizations' crimes? It only follows the same logic that the Blades themselves set down. It's arrogant ignorance to think that they can use this logic to get me to kill Parthurnaax and not be subject to it themselves.
3. They are a secret organization, if you don't become one of them why should they share their secrets and knowledge with you? I can't imagine e.g. CIA letting and unauthorised personnel enter their HQ.
They're so secret that everyone has heard of them and their roles in the past, right? just because everyone thinks they're wiped out doesn't mean they're still a "secret organization", there's just two of them and they dare not say anything because they're so inept that they can't do guerrilla tactics against the roving bands of Thalmor. Nope, they're just perfectly happy wallowing in their own misery and fear to the point that even the
memory of the Blades is an incompetent joke.
4. After you recruit new Blades they help you hunt Dragons... if you wish so.
And they help you about as much as Delphine does. Which means they get in the way and prevent you from performing good maneuvers against whichever Dragon you are currently fighting. They're no better than cannon fodder when fighting a Dragon, and Delphine's comments about you (the
Dragonborn) needing the Blades to help you kill Dragons is laughable. From my memory. I've killed
every. single. Dragon. that our little party has ever come across. I've killed many Dragons
before you ever decided to send me on four wild-goose-chases, and I did it solo. Recruiting new Blades is lower on my concern list than tomorrow morning's BM.
We know for a fact that they are considered dangerous and a threat to the Thalmor. Claiming that they are bad and evil is pure imagination. Also after you kill Alduin Esbern literally bows to you in gratitude and says that we should speek with Delphine asap because she's worried about us. Yup they are ingnorant and arrogant... not

.
Yeah... both of them. The two of them do
nothing without the player, and even when the player is around they're more of a hindrance than such a great danger. Aside from the goal of complete eradication, the two (2) surviving Blades about as dangerous as a wet sneeze. So that reasoning doesn't fly, and I have read all of the in-game lore. the Blades were a danger
at one time, but not any more. And as far as that docier you find in the Thalmor embassy, that's incorrect info based off of old info. Read the other stuff about the fall of the Blades. The Thalmor may still be hunting them to complete their destruction, but the Blades are a laughing stock to the Thalmor.