Zilean offers very little to team fights. If you have a team full of big damage, and 1 stun/support, you might get to sqweak by with a Zilean, but if there is another full support hero picked Zilean just nerfs your team.
The trick is that anyone you would save with your ult is valuable enough to save and can turn things around with the save. Also to be able to snare enemies and speed up allies to get quick kills.
Zilean is one of the best when it comes to team fights.
Before it even happens, Zilean allows hit and run tactics with his haste/slow
He is the master of chasing and ganks
The only thing he doesn't do in teamfights is kill, but that doesn't matter to me.