I bet if you did a survey you would find 600.5(A) is largely ignored. It has been in the code, virtually forever (at least the early 70s) and I am not sure I have ever seen a sign circuit with no other outlets. Granted I did not do a lot of "retail" but "commercial" covers a lot of ground.
I can guarantee the strip mall bay we rented when my wife had a store did not have a dedicated sign circuit. The sign was on with the overhead lighting in the front part of the store because it all came on with the same switch. (four 48" lay ins and a sign with four F40s in it as I recall).
