Who would buy coins (or mine) on a unofficial Zcash network? I can't imagine many people would expend resources supporting an unofficial network since one of the major risks of a zero-knowledge protocol is that the initial operator could "premine" a large quantity of coins and no-one would know.

People will probably wait for the official Zcash launch because they trust the Zcash team to launch a secure network and (importantly) to maintain the network going forward. In some ways this is like a Schnelling point, where people will wait for the official network because their expect other people will wait for the official network, and so on.