Neuro and I aren't trying to imply Uriel is evil, but think of him as a far more powerful and slightly more benign version of John Travolta's character in Swordfish, especially as far as the "You could cure all the world's diseases, but the cost is killing an innocent child.  Could you kill that child?" question Travolta poses goes.  Uriel is concerned with the GREATEST good, not just the greater good.  Given that he exists on multiple levels of reality simultaneously, has intellectus, and has watched the rise and fall of literally all of humanity, there's absolutely no way we can truly understand his motives.  Think about what Uriel said about the Fallen that whispered in Harry's ear.  Something that Harry (and most of us, myself included) missed there: Uriel has that same advantage, as far as understanding humanity goes.

As I said before, Uriel is as close as Jim can come to acting personally within the series, which is the same thing as saying Uriel kind of is Jim.  Assuming that Uriel has entirely benign motives, or that he's acting with only Harry's best interests at heart is worst than silly, it's verging on willful foolishness.  While Mab would throw Harry away in a heartbeat if it was necessary to fulfilling her purposes as the Winter Queen, Uriel is far more dangerous.  He'd likely allow Harry to become the scary evil tardbeast Harry is so afraid of becoming, IF it served the greatest possible good to allow that to happen.  Simply put: Uriel has supported Harry in the past, and has helped him, yes.  BUT (in huge flashing neon letters, that does not by any stretch of the imagination make Uriel Harry's friend or ally.

Neurovore is right there.  Uriel and Harry are operating on different planes of existence.  It's like that conversation in Flatland when a two dimensional shape is speaking to a three dimensional solid.  Harry can't comprehend what's going on with Uriel, because his brain isn't set up to do so.  Plus, Uriel is as close as Jim can get to actually being in the books himself, and we all know how Jim delights in not telling us things.