Those are interesting points and I pretty much agree with you. I'm not convinced glycogen depletion is all that important for our purposes. This is sort of a shot in the dark, seeing as how I don't have an exercise phys. textbook and I'm too lazy to dig up references. But I think a preworkout drink is going to give you an increase in insulin, increase in glycogen synthesis, along with all the other usual physiological effects. The onset of exercise is going to see those insulin levels go down, while non insulin mediated effects at the working muscle and liver are going to see blood sugar go down with possibly hypoglycemia as a result. (especially with the increase in carb oxidation from the the sudden rise in 'slin and CHO preworkout) I recall the Barrbary Pirate saying something about using table sugar to keep blood CHO up, and that seems reasonable.
The best part is as the adrenergic effects of exercise subside and the stomach empties, along with insulin levels coming back up, you're going to get the effects of the preworkout drink, including, according to a study I can't recall the particulars of, better utilization of subsequent postworkout protein. That leads me to say that, as you suggested, preworkout protein alone would also have benefit.
I thought I read somewhere once that the rate of change of CHO or protein concentration was perhaps more important then the absolute concentrations, probably above some threshold of neccesity of course. I would think, without looking at any data that you're going to see larger, faster changes in concentration with the preworkout drink, perhaps because GLUT4 expression is increased from exercise during the precise time when the gut's given' up the goods.
That was a hodgepodge of crap, I hope some of it made sense.