If I were to build a PC, I don't want to build a measly $600 one. I would want to build something at least worth $900. This way though, I have a baseline for what I want, and I can upgrade as necessary. The main pro of this is that the case is ginormous, so throwing in a lot of RAM and a huge video card won't be a problem.
Though if I ever want to switch processors or upgrade my RAM, I would have to get a new motherboard entirely. The existing motherboard is fine as it is right now. I would only need to switch it if I wanted more RAM.