Sure, it is disabled when you last hit. Except when you finish it, then it doesn't get disabled anymore. Plus you get 60 AP, +50% base mana regen, 10% cooldown reduction, not to mention a 2 gold per 10 seconds that isn't lost ever, even on the lower tier versions.
The upgrade to tribute when you reach 650 gold with it is also very nice, especially on immobile casters such as Annie, Cassiopeia, Morgana etc, because it gives you the ability to reposition up to 3 times during a teamfight, and kite enemies much easier.
And that's before even starting on the active, giving you vision of the two closest champions, and if used right, gives a 5 second slow on them.
So what's the problem with it? As far as I can see, it is a heavily oversaturated item that gives you so much more than any other mage item can give you. Why don't people use it?