I think the design of the Mac Pro is pretty cool, but I am just pointing out that a computer with an open hole for ventilation like this will be prone to foreign things like liquids and particulates. It is a perfectly valid and logical concern.
Most modern PC cases have air vents at the top. Heat rises, so the most efficient means of cooling is to push the hot air up and out.
My PC case has a giant mesh grate at the top, directly above the horizontally-mounted motherboard, where I can optionally mount some huge fans.
It's not really the manufacturer's concern for what spillage accidents could occur. Those are user error.
This Mac Pro is supposed to be as quiet as a Mac Mini (which is pretty much inaudible), so I'd say they did a great job with the heat dissipation, especially considering the power they've packed into that thing.