You clearly don't know how Invisibility works. I'll give a run-down of it:
1. You cast the spell and get a 5-second buff which reduces the threat you've generated by a sizable chunk every second. Basically, a nice way to drop aggro quickly.
2. At the end of the 5 seconds, you become invisible. This is INVISIBLE, not stealth. That means that you cannot see ANYONE unless they are a) in your party/raid b) also invisible, or c) Have invisibility detection (like the Warlock buff)
If any kind of action other than movement is performed during the 5-second threat-dump buff, then the buff ends and you don't go invisible. Literally, anything other than walking. Using items, casting spells, anything. Also, -any- action performed ON you will remove it, except possibly heals. DoTs, spells, hits, anything, even if resisted, will cancel it.
It isn't a stealth. It isn't that useful in PvP. It costs a fair bit of mana for such a finnick thing. And it costs you quite a bit of time of being unable to do anything. Plus you can't just PoMPyro from it, since you can't see people.
There's a reason there have been a lot of mages (myself included) that have been calling for a change to invisbility mechanics, and asking for the 5-second buff to be shortened by half or more
Edit: Oh, and I forgot to say, LOTS of things have Invisibility Detection in PvE, often something found out -far- too late.