Zilean is worthless without any farm. It's not like he's hard to play p/2453168
His kit in early to mid game focuses on harassing and bursting with one or two bombs. When he gets some AP he deals enough damage with one QWQ rotation to clear the wave, and roams. He provides a lot of utility in ganks; imagine the AD carry getting 7 seconds of FREE DAMAGE because she's not being focused since you used Chrono Shift on her.
His kit in early to mid game focuses on harassing and bursting with one or two bombs. When he gets some AP he deals enough damage with one QWQ rotation to clear the wave, and roams. He provides a lot of utility in ganks; imagine the AD carry getting 7 seconds of FREE DAMAGE because she's not being focused since you used Chrono Shift on her.