Thats just a power fantasy play, because people like to see the big LB attack finish the boss. It doesnt bother me at all when a dungeon run is going smoothly. If you have a capable tank, capable healer, good dps, and no dicey moments, then it doesnt matter if they save it. Also doesnt matter if its used during a pull.
When it does matter is when things are looking iffy and people dont use it.
I dont get why people would complain about it being used during pulls. Outside of someone not knowing the mechanics during bosses, its the pulls that pose a risk and where groups are most likely to wipe.