Spoilers and all that, but there's a boss with 55 DEF you have to get through (on your own if you don't go the lewd route) - you'll know him when you see him. Pierce, IMO, is a lot more viable than Brittle for that battle. Not only that, but as just about all enemies have some measure of DEF, you're going to do more damage with Pierce than you would with any standard attack. Sometimes it's not necessary, but the extra damage may save you a round or two of fighting or could even be the difference between dealing damage and not.