let me answer your question with another. why would you? i know i know there are of course reasons for everything. run your servers how you want, dont let anyone tell you otherwise
-the old grey beard admins here say back in the day they plopped the OS on a raid array and make the rest of the drives standalone to save money. these days with NAS's and SANs and inexpensive drives for RAIDs i dont see the point. they still do it because thats how they've always done it and will always do it
-windows updates and av updates, limited size means limited ability to do updates. some of the older servers i run have 50 gig drives, and you have to do updates in packs of 10 or else you end up out of space.
-there are a lot of apps that want to be on the OS partition like a child that doesnt want to leave home. even if you install on the other partition you still have junk installed on the OS partition.
-ive heard other people give me some lame answers like a supposed increase in speed or something to do with fragmentation but i dont believe any of that nonsense. ill believe that when i see it.