It's just not as easy if there's no network connection. Install your benching OS, and forget you need network for .net 3.5, and it's a pain in the ass.
Also, trying to install .net 3.5 on Win 8 from the deploy package is a no go from what I can tell. You get the same message as when you try to run something that requires .net 3.5
Probably including .net 3.5 in your unattended Win 8 install would be the way to go if you're benching XTU a lot.