Look, I'm not a network engineer, so I know nothing about rack sizes and such.
I'm just pointing out that the Mac Pro (which can be used as a server product due to its nature) is frequently seen in server rooms in various types of rack systems, perhaps not the 19" you refer to though.
What you are missing though is the point I've made all along which is far more important.....
Where exactly are you going to put those dustbins in a server room? Just leave them about on the floor? You can't put them on their side because of the design, plus they also have a fan which is based on upwards movement, which will cause an issue for other server equipment where the air is expelled horizontally. Finally there is the fact you can't stick RAID cards and such inside the new Mac Pro.
Don't get me wrong, the new Mac Pro is a behemoth. However, much like nearly everything that Apple creates under the guise of 'Pro' these days, its now squarely aimed at individuals who want power, rather than businesses.