The rackmount Mac Pro isn't really a server, it's just a Mac Pro in a rack mountable enclosure. It's 4U and kind of poorly laid out for a server. It helps if you need 10 or 20 or a 100 Mac Pros specifically and have to position them in an efficient manner but a proper server would be no more than 2U with a very compact layout and little wasted space inside so you can pack a rack as densely as possible.