Re: Stupid U
"Also, I suspect the vast majority of cloud datacenters today (including those in Facebook, Google, Microsoft, Amazon etc.) are using 19-inch racks.. So deploying 21-inch racks only becomes a consideration"
The majority of cloud data centres typically deploy equipment as a pre-built rack - ether during initial deployment or during equipment upgrades to allow for pre-deployment testing so it's unlikely to be a significant consideration outside of the physical space required.
As the majority of clouds DC's are also power limited (i.e. either directly or indirectly via cooling limits), space is unlikely to be an issue.