Pretending to fall to a blow is a perfectly valid tatic.
Now picture this. NPC's round: take full attack. PC decides to fall after the second attack, to prevent futher damage. The PC's turn comes up next, and he stands up and attacks the target. If that PC is a rogue, they should get sneak attack. But, they never spent a round to perform the action of bluffing. They get sneak attack for free.
You might say they shouldn't get sneak attack at all. I would say this would make an excellent feint though. I am not going to pay attention to a person who I think is unconcious. Hence a skill check.
I most combats I have seen in D&D, you want to make sure opponents will not attack you again. So, if a person I was fighting were to fall and I noticed they were not unconcious, I would keep attacking them. If they fell, they are probably close to dropping unconcious anyway. If there is a more powerful foe, I will finish my current opponent to prevent him from aiding the more powerful foe. Dropping to the ground isn't surrender, after all, it is a deception.