Those are some points I have been concerned with as well. About the only way I could see them being used effectively is if the party leader can choose who uses their Limit Breaks. If anyone can use theirs at any time I could see some serious issues in Pick Up Groups or Duty Finder Parties. And if everyone has to confirm the use of one, they would rarely be used at the right time (WHM and PLD especially, assuming they have a healing/invincible LB).
Edit: Bleh I'm out of it after work, can't even type the correct words.
Edited, Jun 6th 2013 5:16am by DamienSScott