If I understand your post correctly (which I might not!), then actually you're wrong about the hydrating forces. It's not osmolarity that drives the water into the cells from these drinks; indeed, under normal circumstances it'd pull water out. Remember, hypertonic solutions pull water towards themselves.
Gatorade (Na/sugar/water) does "trick" your body into wanting water, even if cells are plentifully hydrated OR if other mechanisms in your body, natural and healthy or foreign and/or toxic, are pulling water out of your cells. It does so through the Na+/glucose symporter (I had momentarily forgotten the name of this in my first post), whereby a Na gradient favoring Na entrance into the cells pulls it and glucose, along with the water solvating it, into the cell. During exercise, intracellular glucose concentrations are depleted, so this provides a further drive of nutrients into the cell, pulling even more water into the cell. This is why you give patients with diarrhea gatorade and not water to rehydrate them.
Other points about the necessity of electrolytes are true and important to keep in mind. Hope this helps.
For the sake of brevity I'll copy this from wikipedia (I know). "Osmolarity and tonicity are related, but different, concepts. Thus, the terms ending in -osmotic (isosmotic, hyperosmotic, hyposmotic) are not synonymous with the terms ending in -tonic (isotonic, hypertonic, hypotonic). The terms are related in that they both compare the solute concentrations of two solutions separated by a membrane. The terms are different because osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of only non-penetrating solutes.
Penetrating solutes can diffuse through the cell membrane, causing momentary changes in cell volume as the solutes "pull" water molecules with them. Non-penetrating solutes cannot cross the cell membrane, and therefore osmosis of water must occur for the solutions to reach equilibrium.
A solution can be both hyperosmotic and isotonic. For example, the intracellular fluid and extracellular can be hyperosmotic, but isotonic - if the total concentration of solutes in one compartment is different from that of the other, but one of the ions cannot cross the membrane, drawing water with it and thus causing no net change in solution volume."
Na+ ions do not readily cross the plasma membrane into the cell due to permeability and concentration gradient outside Vs. inside (not going into propagation of action potentials). Therefore, there would need to be a pressure gradient via plasma volume vs. cytosolic volume and its constituents until an equilibrium is reached. When extracellular volume increases with slight oscillations in [Na+], diffusion of water is shifted into and out of the cell to maintain cell hydration and plasma volume. During exercise, depending on many factors (intensity, duration, fatigue state, substrate availability, etc) will determine if muscle glycogen will be utilized and to what extent. Mechanics behind that are beyond this topic. Plasma membranes are also not readily permeable to glucose (except during exercise where permeability is selective due to sheer stress). Otherwise, a glucose transporter must be translocated to the membrane for glucose transport into the cell.
P.S.: The purpose of adding electrolytes is to restore lost Na+, K+, ~Cl- ions from sweating during exercise which aid in normal cellular physiology.
