As an experienced WHM, I like to make sure everyone has SS up at the beginning of the run. I'll recast SS on DPS if they lose it during the battle and then I swiftcast it on the tank while we're all on the run so people don't have to wait for the cast time. The only bossfight I use it on consistently is on the Ton King. I like to time it so I cast it right before he does his super stabby. It's generally unnecessary for most tanks, but it keeps WP interesting for me.