Usually going to be a LNC, PUG, or THM (or their equivalent jobs, of course) that use the limit break. People want the burst damage versus tank survivability or extra heals. Of course, you should always adjust to the group - if you have a tank that takes a lot of damage on boss fights obviously extra damage isn't so useful when the tank is dead.
Full party groups are different in raids, however; situational use of LB is important. For example, during Ifirit HM people want to save the triple LB for a BLM aoe nuke on the spikes. Coil raids might want it saved for WHM aoe rez.
The only guarantee you've got really is that a Bard will never use the LB, lol!