I run with a PAL who like Cover (almost never see it in PUGs and DF though, even when it could be used). For the most part, it isn't needed, but when is its a lifesaver.

WP runs are where I see it most, especially when pulling 3+ groups at once. PAL pulls everything, Covers WHM, WHM spams Cure II. Other times if the WHM doesn't need Cover, it goes to the BLM or BRD (whoever pulls more aggro at the time). He also likes to use it during the Pudding boss, but honestly, I don't think it actually works on the AoE...

Outside of WP...its really just whenever someone needs it; caster or heal pulls too much aggro, gets Covered, fight goes on.

I've also heard its VERY effective on Titan HM. MT a WAR (or a PAL, really doesn't matter), OT a PAL, use Cover for a Table Flip if the MT isn't topped off.