So, if your "Main Tanking", you should NOT be applying your Slashing Debuff as a WAR. it's NOT worth it in your rotations. If you're "Off Tank" on a single boss, then you should ALWAYS be applying both of your Debuffs. When you're MT'ing, tell your Ninja to not be a bitch and apply the Debuff whenever it comes down. The slashing debuff can buff 3 whole classes, PLD, WAR, and NIN, thats a nice boost across all three classes, which adds up to a nice chunk.