Remember, you can do a defensive switch so he's not guarding the opposing PG
You can do that, but that would mean that you'll need say a PF with as good OD as normal PG. Now that is not very likely to happen...
I also think that IS plays a role to an extent in driving layups but I don't believe it's essential. There are guards with low IS who can make driving layups frequently. And besides, majority of the guards take shots from the perimeter as opposed to from inside. So training IS for guards is not very essential imo.