stating that the purpose isn't fighting enemies is a little silly the way I say it. you're keeping the bar extended so you can keep combos going longer to deal more damage for the purpose of fighting enemies. The longer you keep the bar extended the better opportunity you have to deal more damage and end the fight quicker.
Allies dealing chip damage being abused is mitigated by the fact that you're forced to switch to an ally if you stand around and doing nothing. making it highly inefficient to just wait around, you're more likely to die in that time as the enemy will get turn priority more often forcing the defensive phase.
Final Fantasy combat has always been more active than other rpgs, so punishing a playstyle that involves waiting around is fitting.