It's not magic.
You still need to track what you've got and monitor your systems so you stay ahead of your customers' requirements. Just means that most of the time you only need to buy (and power and cool) half the disk that your Windows boxes think they need.
Just make sure when you buy your array that the thin provisioning they implement allows you to overprovision should you choose to - not all of them do. If not, you're either left with half a rack of disk which thinks it's "full", or a full rack with no thin volumes. Genius.