I am still very new to the game, but when I read up on limit breaks, my impression of them was to use them when it is called for by the tank (or other party leader) since it is not an individual action. Obviously, at the time I had no idea how dungeon runs go in this game, how LB mechanics work, or the rate of use which is considered the norm.

Now that I am a little more acquainted, saving it to overkill the boss' last sliver of health is definitely a waste. However, I am still not sure LB's were designed to make dungeon runs go faster, OIOW increase dps. In either case I can sure in hell tell you that dps is going to go WAY down if you are arguing amongst your comrades about how to use the LB. I would probably kick myself from a party with members gouging each other's eyes out about it.