I used to be a server monkey long long ago, and played with the mockups of these servers that were set up at FB HQ. One thing to realize is they are heavy. Loaded with hard drives, one of these servers is roughly the size, shape and weight of a long 120mm mortar shell.

Easier to rack than the Dells of yesteryear, sure. But there can be up to 120 of these bad boys in a single standard rack, which means 60 of them have to be lifted above waist height during rack and stack. That is pretty strenuous, and technically you should use a moving lift and / or a lifting partner. Especially when you're talking about hundreds or thousands of racks.

Thinking outside the box and keeping the poor floor staff in mind, I've always wondered why we don't build little catwalks or half-height rack units, or even moving racks, like a dry cleaner's.