Omg, right?
Soo, I was just working at a ferry terminar, as car marshal, and there was an actual mini blizzart rolling around! This is amazing! The mini-blizzard just improved the atmosphere, and I really enjoyed doing it, despite almost having my toes frozen off...

And, yeah, the inner shepherd dog inside me was super happy to be shepherding cars and trucks, and did a good job at it. They even let me regulate all the traffic all on my own, after a while! And the best part is that I got payed good money for the whole shift, as well!
@Cheza
The problem is just that they allow Lady Serana to run around freely despite her being a vampire who randomly showed up at their doorstep one day.
And she's not just a regular vampire either, but a pure-blood vampire of clan Volkihar and the daughter of an extremely powerful vampire lord with a god complex.
Makes even less sense for them to let her roam free.
If they can work together with a vampire that's "always" been a vampire, why can't they keep working together with me?
What's to say she isn't going to turn on them eventually? How can they trust a pure vampire more than a vampire hunter who has proved himself time and time again to be dedicated and reliable?
You're saying it right, there...
They allow Lady Serana
to run around freely, because she's not an ordinary vampire, but has proven herself as un-hostile for the time being. But, still, she's not admitted to be scheduled into dawnguard shifts, or onto their terrain, because she's a vampire.
Exactly as they allow you to run around freely, and won't murder you on sight, because you're no ordinary vampire to them, but you're a comrade who's proven himself well, but has unfortunately turned into a vampire.
Now, the reason why the dawnguard doesn't mind working together with Serana, is because they're actually not.
You've said it, she's a full-blood vampire, and daughter of Volkihar's clan leader. So, she falls under the Volkihar command structure, and not under the dawnguard structure. The reason why this matters, (Besides the dawnguard potentially knowing that Volkihar FOR are trustworthy) is that if she, at some point betrays the dawnguard, then they can blame it on the Volkihar command.
You, whilst you're not an ordirary vampire, don't fall under any command structure at all, let alone one that the dawnguard is allied with, and the dawnguard won't allow vampires in their rank, soo they cannot assign you to any orders.
And, if they did, and you [REDACTED] up on their payroll, they'd have noone but themselves to blame for allowing a vampire to act under their colours.
But, because you're their friend, they're letting you go your gang, and are not killing you. They're not even giving you a permanent medical discharge, because you've been infected with the vampire disease. Nope, not even that. Prove that you're cured, and you're free to come back!
Still makes sense to me...