First of all, you shouldn't compare Mac Pro to a regular desktop as it's a workstation. However, Dell offers in-home service even for regular desktops. For workstations, they offer pretty much anything you want.
I know and I completely agree with you. But, the OP (and many others with this question) compare it to a desktop. I'm only trying to offer a suggestion as to how one can justify the MP price when 'only' a desktop is required.
But, I did not know that Dell offers in-home service. Is that free? Seems like a great deal. I've had experience with HP service second-hand and it wasn't good. But that was with a laptop so maybe they have different policies than a desktop.