I think we are omitting one other possible action by Uriel to give aid to Harry for the twice hell-blessed power Luci gave to his minions. As Kringle says, knowledge is a power like any other, and one of the most pure and dangerous forms of Power. Uriel''s conversation with Harry, and the gifting of a book filled with wise morals and good philosophies, of good overcoming evil and the common man rising up to challenge the darkest of foes, could be the second favor.
I don't see Uriel doing anything that cannot net him some gains in the future. Harry was having a crisis of Faith and Uriel uses the marker Luci played to save a good man from forsaking his Faith. Harry was at the point of saying F-U to the TWG when Uriel arrived in humble form to help guide him to a better understanding of the situation. Wrath, I would say, is Harry's strongest vice. To help a man stop from succumbing to it and have him lose Faith, seems like the best use of a marker.