I think it has more to do with the YED's game plan and finding out the endgame. Sam is the only YED victim left and he's the only one that can provide any clues to what the YED was planning.
, but because of the way his secrets, lies and powers are driving a wedge between Dean and Sam and therefore making it easier for the angels to get closer to Dean.
Not exactly, I think, there is Rosie (and her generation), too. The angels would just have to wait for about twenty years. ;)
But the real reason I wondered about their good cop/bad cop dynamic is because in "In The Beginning" Castiel said (if I remember correctly), "You [Dean] have to stop him [Sam]. If you don't, we will." And Dean shot him a death glare. So ordering Dean how to deal with his brother, especially with the added threat, wasn't exactly a good idea, if Castiel wanted to get Dean on their side because at that time Dean was completely on Sammy's side and didn't know anything about Sam's doings. Dean didn't like it when JOHN told him that, he sure liked it less when a (not so random) angel was telling him. But after Dean's glare Castiel's behavior changed a little. Gone are the threats ("I pulled you out of Hell, I can throw you back in.") and the orders ("You have to stop it/him.") from the first three episodes, left is a Castiel who is trying his best to make nice with Dean and goes so far that he admits to having doubts himself which, honestly, he maybe shouldn't have told anyone about, even if he really has them because of, you know, being an angel without a falling wish
. I'm not saying that Castiel isn't the friendlier angel of them but that Castiel is trying a little bit too hard to get to Dean while still not doing anything to get Sam and Dean back together.
Now in this episode Uriel suddenly shows up and takes over the part of the bad cop with the Winchesters, something that Castiel did in the first three episodes without hesitation even if he was mostly friendly back then, too. Maybe Uriel does so to show Dean (and Sam) that Castiel really is there to trust for Dean and understands (or rather wants to understand) human nature. Uriel doesn't try to make nice with Dean at all, he doesn't hide that he just cares about getting the job done. And if he can smite a town full of people that's just a bonus. Castiel next to him has to look like the good guy to the Winchesters. Uriel tells/orders Sam that he should better stop using his powers/hanging around with Ruby (by the way, I agree with the angels here), something that Castiel told/ordered Dean before but has stopped in favor to emote about his own touchy-feely self-help crap (to say it with Dean's words :) ). Uriel isn't nice to Dean (4.10. and the next threat of throwing Dean back into Hell), he isn't nice to Sam and he sure isn't doing or telling Dean anything about his supposed mission except that his brother is bad, bad news.
Furthermore, if Sam doesn't use his powers, if Sam stops with whatever it is the angels want him to stop, how is Dean (or how are the brothers) supposed to find out Azazel's endgame if their only clue is supposed to be Sam using his powers and running around with Ruby? It just doesn't make sense to order Sam to stop using them in that case.
IMO, it would have been far more sensible to sit Sam and Dean down together and tell them they need to find out the YED's endgame, so could Sam, please, play along with Ruby as long as they need him to and could Dean, please, stop fighting with Ruby and instead try to find out what she knows and could the boys, pretty please, work together?! Because it's pretty impossible for Dean to really find out what Sam and Ruby are doing if he is fighting with Sam all the time and if they don't trust each other and if he calls Ruby a bitch and doesn't try to gain her trust even if he still hates her guts. But that's exactly what is happening now and everything Dean knows about the YED's endgame is that Sam is sneaking around with Ruby IF that's part of Azazel's endgame. That's not exactly much, I think. The way the angels are dealing with Sam and Dean doesn't really help Dean's mission if this is Dean's mission. However, if they are trying to set the brothers against each other (or just to push them farther than they maybe would if it wasn't for mendling angels and demons) because they can't have Dean listening to Sam when it comes to his real mission whatever that may be, then their plan is definitly working with all the distrust, lies and secrets between the brothers now.