This is due to a defect in the grid where we are blindly overriding the getRowClass function on the viewConfig w/o checking to make sure there wasn't one there already. Hopefully this defect will be fixed soon. Check out my answer to this other, very similar question: https://stackoverflow.com/a/17891138/728184
You should be able to have your getRowClass function win out by setting it in the beforerender event listener (thereby re-clobbering the one that we put on there, which is really only useful for automated testing anyway and not required in any way for the grid to function correctly).
UPDATE:
I just fixed this in the nightly build, so this should no longer be an issue in public sdk builds beginning with the next public release after 2.0rc2.