See. I think Michael was allowed free will. And he could use his free will with the Grace, while Uriel couldn't even do that. Therefore, I see that Michael having the Grace and being in that position has more flexibility than had it reside in Uriel. So, Uriel counted on achieving more with it than he could have done by himself. Of course, the immediate use was to heal Michael so as to allow him to intervene successfully against Nic. I think though Uriel needed to have his Grace on the mission to Hades realm.