Back in 2006 a few of us predicted that developers wouldn't take the bonus because they'd find enough bulk without bothering with the Inclusionary Zoning bonus. That's because the city actually allowed a loophole in the zoning large enough to shove a buiding through it.
The zoning allows far greater height than the base bulk allowance. So if a developer can buy air rights from an adjacent building, the developer can fill the height with luxury units and not bother with the affordable bonus.
If the city had modulated the height caps with the bulk allowance, developers wouldn't be able to take advantage of air rights. But since the height caps are fixed and in excess of the base bulk, the city invited developers to buy air rights, encouraged it, welcomed it, pointed to it and ushered the developers to it.