The idea behind small PC's is that they are easy to move around (lan for instance is a joy with one of these compared to a normal case) and are very easy to 'hide away' in normal use, ie. they don't take up much space.
A lot of people (myself included) like them, I've lived with the grey standard cases for years, and I'm beginning to get tired of them.
Maybe, and just maybe, the next computer I will put together will be a really small one, but that depends on my funds, otherwise I would have already built one!
By the way, I saw one of those shuttle cases with a 120 mm. fan at the back, blowing outwards on the extended heatsink, but that may just have been a mod. I didn't take a closer look. It does prove that it is possible though.