I think the point is that a car pc can do all the normal things like sat nav, but a whole load of other things that can't be done without a PC. Yes, you can use a laptop but have you ever tried using one in a car? It's not very comfortable! I suppose PDAs are ok for email, but their functionality is severely limited. Surely it's much easier just to sit back and dictate an email to an onboard pc than use a PDA.
I don't see how having a PC screen in a car is any more distracting than any other type of screen, be that a separate sat-nav or a factory fitted system. As detailed on the above page, it says you can't do stupid things like surf the internet while the car's moving, so the fact that it's a PC is irrelevant. Are you going to ban all sat-navs? I think sat-navs increase road safety because drivers are less sressed, and they don't have to keep looking down at a map.
You also said sat-navs are going to be faster. Really? I don't know what processors sat-navs use but I'd put my money on an Intel Core 2 Duo chip performing a lot faster! Also the PC has a much bigger screen than portable navigation devices, which makes it safer because it's easier to see, and much more usable too.
Obviously not everyone is going need one, but I can see plenty of uses especially for businesses with sales reps and engineers on the road. By the way, do you know which cars in the US have the Microsoft operating system you talked about? You're not thinking of Ford's 'sync' are you, which is made by Microsoft but is just a glorified phone kit and music player, not a proper PC.