Once again there is a lack of understanding. I don't think anybody was talking about grounding or two wire vs three wire cords but since you ask- There are no metal parts on a light string that would need to be grounded. It has a two prong plug and there is no reason to use a grounded extension cord unless you want to. Nobody is saying that you should. A two wire cord is fine. The only thing I am saying is that the light string, the plug and the extension cord ends should be made to be water tight which an indoor extension cord is not. Then you have no shock or leakage problems and no problems with GFIs. There are extension cords made for construction sites that have booted plugs and ends as well as the power distros that go with them.
The only things I can think of that would need a grounded connection is a display that has metal parts. Once again they should be listed for wet locations as well as the three wire cords connecting them.