Kind of strange to refer to "laptop like performance" considering the parts he's using.

And why not? Aesthetics are very important for a lot of people. Unless you're doing some sort of watercooling or need multiple GPUs or other expansion cards or need to put it a bunch of HDDs there's no reason you can't go for a smaller form factor. Why should a mid or full-sized ATX case be the standard?

The problem is that high performance builds produce a lot of heat, and those tiny cases do not have enough space to let that be easily extracted with low RPM high diameter fans, so you are going to end up with a pretty box that is going to be pretty noisy, unless you stick to lower power consuming and thus lower heat producing laptop like parts.

Space, I suppose. And those of us w/ Mac or design backgrounds appreciate a smaller and more elegant computing... (not for everyone, I know.) There is a point in that most users who aren't using huge multicore or GPU workloads shouldn't have a giant box of fans occupying half the legroom under the desk, esp in smaller work environments. And yeah, they're missing out on high end capability but the form factor is more appropriate to the use-case

At the other extreme are those folks who work on the iPad exclusively, like the poster/blogger who just runs everything on the cloud using a SSH client.