There's an interesting flavor to it: you stand over your ally and shelter him so he has more flexibility in how he gets to his feet.
t~
Actually the wizard (yes, the wizard) in my gamming group did that. She steped inside the fallen cleric's square and shouted at the enemy: " you will not step any furter". I remmembered Samwise quite to Larachna... amazing scene.
Player: Can I make an Acrobatics check to stand up in an adjacent square?
DM: Sure can. Let's call it in at Moderate DC.
Player: (rolls) Alright! I roll onto my feet and swing my weapon upwards as I go! (rolls attack)
Ah, Acrobatic Stunt, you're so awesome...
Hahahahah Claudio, trying to sneak in that exploit again?

You and I know you were trying to abuse my friend... double run, fall prone, and stand up free next round keeping the +2 bonus from prone instead of granting CA to those archers were a huge abusive maneuver....
And frankly these "skill fixes" are terrible, becuse basically the acrobatic character would be immune to the prone condition (as written) as his skills progresses. Makes no sense to me if there are powers/itens/feats outthere that you can buy exactly to do that trick.
Any "fix" would not only weak many knock prone powers, but make those aftermentioned things useless. Wanna get up easly? Spend some character resourses on it and stay a unique character.
Honestly I can underestand why the "stand up" is a well defined move action. And why everyone thinks they have to attack with their best tactic/weapon every single round of their carreers?
Only argument I believe is interesting on all this is that you can shift if there is someone in your square, but you cant if there is noone there. If you
really wanna fix this we can merelly forbid "square invasion". Its way better that give free movement, or create basic combat maneuvers (partial charge) that counter encounter powers.
As aways just my 2 cents.
