Comment below rating threshold, click here to show it.
I know it's an old and necro'd thread, but nobody really answered the original question.
Zilean is rarely seen as a support for a few reasons:
1) His bombs, while excellent harrass, will push the lane like crazy (if the opponent understands how to handle zilean bombs) and probably steal CS from the carry. It also means that even in a matchup you're losing, you'll be forcing your carry away from the tower and into vulnerable ganking territory.
2) His utility is relatively limited compared to other supports. His speed buff/slow is excellent, but it's only single target. His ult is excellent too. But these two spells on their own don't really come close to matching what a typical support pick brings to a team.
3) He's mana hungry and needs AP and CDR if he wants to be a real threat late game. To do this, he needs farm. This isn't something a support usually has a lot of. Without this farm he's largely ignorable aside from his ult.
Now, it's not to say that Zilean cannot be played as a support. Certainly, his utility spells are very good. He's more of a 'fringe case' support as I like to call them. They CAN support capably, but probably should be played elsewhere to get the most out of them. Like Gangplank and Orianna.
If you were going to play him as support, I would likely run magic pen reds, gold/10 quints, armor yellows and MR blues. You could also run magic pen or AP quints instead if you wanted to go all-in during laning, but the gold/10 is invaluable, imo.