On DRK, no amount of SkS will get you any benefit in terms of getting a skill under Delirium, you get 5 BS no matter your SkS (even at 0). On PLD, you could get Riot Blade under FoF at a certain level, but it's not really worth pursuing it because of how terrible SkS is on PLD elsewhere.

GNB, not entirely sure. It throws you off from No Mercy, but it might still be worth it with the increase in gauge.