I was looking for a thread that may have shed some insight on this particular ability but I couldn't seem to find anything on the forum that was related to the topic I wanted to discuss.
So a while ago (maybe like a month ago but I still remember it), I was tanking Titan EX and I was a OT PLD. We wiped a few times but I was in DF so I was okay with it since I was most likely running with newcomers at the time. What got me, though was when I glanced at the MT PLD and saw that his health was around 20% and dropping and so I said to myself "hm, I'll just be nice and hit cover since the boss is about to die and I've never really used the Cover ability before." So, we end up beating it.

The other tank that I covered immediately goes off and swears at me and questions "why the f*** would I use cover" and so I'm like "why the f*** not?" So he goes something like "you must be new or something, that's alright." And so I leave 'cause at that point I don't have time to explain why I used cover to someone I'll probably never see again and continued on with the night. But now it's come back and the question's more burning than ever and so I come here today to pose a few questions that I hope someone with experience can answer. So uh, do people actually use cover? Is it actually useful? Do people actually get offended if you cover them even if they're a tank?