I'd like to think about it from the perspective of the "bubble-maker". I think that shift in perspective might give us some insight into the "bubble".
Why would someone knock an enemy prone, then move one square away from them, rather than remaining adjacent, or moving further away? Note that the attacker has an ability to knock his opponent prone, and the "defender" is simply the target which is knocked prone (and not actually a Defender role).
Obviously, this provides little or no protection from a defender with a good ranged attack. A prone defender can simply attack from the prone position, or stand up and then make a ranged attack. There is very little reason to make use of "the bubble" when fighting a ranged opponent, unless the object is to delay them. In this case, however, it would be usually better for the attacker to simply remain adjacent to the defender so that any ranged or area attacks provoke an opportunity attack. Therefore, it is unlikely that the 'bubble' will be used much, or be very effective, vs. a defender with good ranged attacks.
There will be very little reason to use the 'bubble' against a defender with a reach weapon or attack. The defender can simply get up from prone, and then attack the attacker with reach. There is very little purpose for using the 'bubble' against such a defender, and in most cases, the attacker will be no better off one square away from such a defender than he would be immediately adjacent to them (where he could, at least, get opportunity attacks). It is unlikely that the "bubble" will be employed to devastating effect vs. defenders with good reach attcks.
Thus, the "bubble" appears to be a tactic which is most attractive to use against a defender whose best attacks are melee attacks without Reach. This might strike many posters as obvious.
However, it is also important to note that even against such defenders (those whose best attacks are against adjacent foes), the "bubble" is only employed vs. a very small subset. The "bubble" does not prevent the defender from attacking another target. The defender can easily stand and charge another opponent. The 'bubble' does not prevent the defender from retreating. The defender can easily stand and then use a Standard action to Move or Run. Unless the attacker has threatening reach, the defender is free to move away from the combat.
Thus, it would appear that the "bubble" is a tactic best applied to a fairly small subset of defenders. It is only particularly effective vs. defenders with no reach, who have no or ineffective ranged attacks, who also do not want to retreat or engage another target.
Now think about this new tactical option. Normally, you cannot attack someone, then ready an action to attack them again before they can attack you. However, what if you were able to use a special tactic to "trip up" your opponent, knocking them off balance, and ensuring that, if they tried to attack you, you would be able to strike them BEFORE they could close in on you. You arent trying to get away from your opponent, but simply ensure that, through careful footwork, you keep the initiative in the fight.
Guess what? That's the 'bubble'! Barring an Action Point, there's no way to reflect the (very common) real world tactic of hitting someone when they are down. You simply can't knock someone down, and then hit them while they are rolling around on the ground, or even while they are off balance. You knock them down, and they pop up on their turn. However, thanks to the "bubble", the game CAN reflect this tactic. If you WANT to knock someone down, and stay "on top" of them, then, within the rules, you move away 1 square. This reflects you keeping a wary eye on your downed opponent. They simply CAN'T attack you (barring all of the clever magical/feat/power tricks) before you get to go again and hit them.
Either they back off (or attack someone else), or you get to hit them AGAIN before they get to hit you.
The "bubble" may seem to create an artificial construct in the rules, a safety zone where you cannot magically be hit. However, if you look at it in another way, the "bubble" actually enables a real world tactic. If you do away with the bubble, then knocking your opponent prone is basically a special case, only used in very particular circumstances. If you use the "bubble", however, an attacker can effectively get his 'licks in' on his downed opponent. He knocks them down, circles closely (the 1 square move) and hits them again before they can go (and if he has an at-will prone attack, oh boy!).
Before doing away with the "bubble", think of it as a special tactic just for those prone attacks.
