Well, one must ask the question why the tree roots are exposed. This is pretty common in urban areas with limited top soil and effective rooting depth. This is also common if the trees were originally planted incorrectly.
With that said, the majority of the effective root zone that takes up soil moisture and nutrients is not at the base of the tree, but rather spread out like a spoke in multiple directions from the base of the tree, and further, these fibrous roots which take up water and nutrients are usually in the top 12-18" of the soil. Anchoring roots, which provide stability for the tree, are more concentrated near the base of the tree.
As long as the the "effective" rooting zone hasn't been buried, then I really don't see a need to worry about the recent build up of soil and grass at the base of the tree. HOWEVER, we don't recommend piling soil (or mulch) where it "touches" the base of the tree due to potential basal rot (i.e., excess moisture is held next to the base of tree thus causing premature butt rot).
Hope this helped!