If GPS units are reasonably affordable for the organisers to hire, then I could see that being useful as a backup to the marshals' count - i.e. do a calculation as normal, then compare to the GPS. If the two match (to GPS tolerance) then accept it and move on. Only if the figures don't match does any additional time need to be given to double and triple checking the counts, with perhaps examination of the GPS log showing where the count is out with a missed/gained lap etc.
But not having helped on the admin side of one of these, I have no idea how helpful or not that would be, compared to the cost of hiring the magic boxes.