Cover is... an odd beast. Sometimes you'll pop it, take some damage, and call it a day. Sometimes your healer(s) will get mad at you because the person they expected to have to heal suddenly isn't the one taking damage and now you're nearly dead. Other times you'll actually save someone's life and feel all fuzzy inside.

The best word for it is probably "situational". For example, Canadane mentioned that as Titan EX gives stacks to the current MT you can Cover that Tank to receive a stack in his place... thereby removing the need to swap Tanks. Your seemingly innocent thought of trying it to see if it would help is absolutely on the right track. As you experiment more you'll find additional situations where it comes in handy.

Just a tip, though: You might want to assign a macro to the action so that people have a chance of noticing that the big obvious line that's suddenly connecting you to them isn't something bad