Zeffi'sZeffi's and TLousky'sTLousky's answers adequately cover the topological reasons creating a face in that spot should be avoided. But I thought I would explain how I would create a topologically-sound face in that spot.
First bevel the edge loop where you want to create the face with Ctrl+B, then delete the resulting new face loop and grid fill the two boundary edge loops separately.
This process fills the top as you want, but keeps the geometry "nice" and manifold with good topology.