While it is true that gravity is on the side of the top player, you're not free of inherent advantages from your guard either. Your mobility is lower because you're pointing all four of your limbs at your opponent, including the two much, much stronger ones. He has to use his legs at least partly to maintain his base and mobility.
This is a valid point, IMO. I also read a quote from Pedro Sauer saying that while the top guy can only ever exert power equivalent to his body weight, the bottom guy has the ability to push off of the ground to exert (probably) even more power. Obviously the top guy has the advantage when it comes to conservation of energy, as the bottom guy has to use more strength, but still, I think it's one of several points that go to show that the "all things being equal, top guy has the advantage" argument is a bit too simple.
This being said, I obviously do agree that everybody should strive to get on top and pass the guard (or take the back if available) in order to have the best shot at finishing the fight.