There's a problem if Flash is picked 9 times out of 10 over any other summoner spell.
How often is Fortify used vs Flash? How about Heal? Clarity? Clairvoyance? Rally?
My point isn't that people having Flash is imbalanced. My point is that Flash ITSELF is an imbalanced spell, period. It's bad design. Crutches should not exist in a game. It encourages bad gameplay. If you can rely on summoner spells to save you most of the time, then you have no reason to improve or to be good.
It would be much different if Flash had a cooldown of 3 seconds whenever you entered or left combat.
Yeah, I can complain about the other team having Flash, but that doesn't mean my team doesn't have Flash either. Flash just causes so many more problems than it solves... if it solves any. Don't believe it does.