Yet, as we where discussing, this limitation of movement is negated by the presence of an ally or an enemy in the target square.
It's a rule that helps adjudicate where you are when someone is in your square. Since they can be in your square when you're prone, if you get up, you both can't be standing in that square. Therefor the rule for a free shift to make sure there's no inconsistency of being in the same spot.
If your ally wants to take the time to get into your square to allow you to shift, then that's a strategy to allow you a free shift, at the cost of their having to position themselves there. It cost something to give you that shift, even if it was small.
It changes the following scenario (Assuming you have the same speed as the enemy):
The last conscious enemy tries to get away. In an effect to slow the enemy down, you knock him prone. On his turn, he uses a move action to get up, and a move action to move his speed and provokes an opportunity attack. Your turn comes around again and you use a move action to get back up to him, and can spend a standard action to do what you need to him.
With a free shift:
The last conscious enemy tries to get away. In an effect to slow the enemy down, you knock him prone. On his turn, he uses a move action to get up and shift farther away from you and move his speed away from you. Given that the target is your speed +1 away, you have to spend both your move action and a charge to reach the target instead of having a choice is standard actions.
That's not the only scenario it changes, but it's an example of how it nerfs the prone condition. If that's what you
want to do, go ahead. But you asked what my thoughts were. My thoughts are that it nerfs the prone condition and the only reason why you should change it is if you willfully want to nerf the prone condition.