in this example http://www.extjs.com/explorer/#editablegrid click at the first cell (row =0, column=0), press tab, and continue press tab.. you can see that the cell that contain a checkbox.. cannot can be access using the tab key...
but... only if is checkbox... think as user... when you will change by tab.. you need get the mouse.. so.. click in the checkbox.. if you have one or two register ok... but if you have 50 checkbox, and when is true.. the user pay your purchase.. you will spend much time to do this operation