Limit Break is special even among unique skills. The power it offers, is beyond anything else, but the cost; is just something that most people would fear to pay. When Limit Break is activated during a fight, it stays active till the fight has ended, only making the user pay once. Limit Break's unique power is to grant the user all of his attack power, in defence, and all of his defence power in attack. Meaning if the user has a maximum of 2000 attack power, and 680 defence power, his attack will now be at 2680, and defence will be the same at 2680. It may look very powerful, and it is, but the price to pay, isn't something in the game. Each time Limit break is activated, the user's real body get damaged.
As for Naomi, she has the skill since she entered SAO, none even her knows why, and she never talked about it to anyone but her trusted guildmates, although people heard of it and speculate rumors. She needed to use it sometimes in order to defend herselves, and other players, even once holding her ground against The Fatal Scythe for more than ten minutes before all the others had retreated, and she followed.
As of now, when SAO is over, Naomi completely lost sight, her whole body is weak, and she has scars on her back and belly just like Sugou Noboyuki has on his eye after Fairy dance Arc. This is what she had to pay for helping in the beating of the "death game" .
Note: In the skill description in her menu, it is written that the skill damages her real body. Just the fact of knowing it, plays greatly on her mental state, affecting her, to the point she's unsure and scared to use the skill, still she does, when she has no choice. She rather be hurt, than die, although she doesn't know what is happening to her body.