Competitive tablets, the iPad and Android tablets specifically, lose a little of system storage to the OS but not much. When you buy a 64GB iPad you have most of that storage space available to add apps and media. It's the same for Android.
That's why the heavy cost in storage space of Windows 8 is a big deal. Sure when you compare the Surface Pro's installation to other PCs you get a similar loss of storage space out of the box. But Microsoft has taken great pains, completely redesigned the operation of Windows 8 matter of fact, to make the Surface Pro a tablet competitor.
It's only fair to compare features like available storage space of the Surface to the competition, tablets and PCs alike. Let's face it, if consumers buy the Surface it is for the tablet capabilities. Otherwise they'd just buy a Windows 8 laptop or hybrid.
The available storage space of the Surface Pro is a clear demonstration to consumers that it's still Windows under the hood with the same impact as previous versions of Windows. That doesn't instill faith in buyers that the Surface Pro is really different like the other tablet choices on the market. It's really business as usual, just Windows posing as a tablet.