One point I haven't read in the contents so far it's:
Building your own system gives you access to higher quality parts ON YOUR PARTS. Most systems you buy will overspend on the CPU, because people buy based on 'the mega hurts' and pull budget away from things that User doesn't think about, like the power supply. Nearly every single issue that a nerd needs to tech support around family and friends comes down to a piece of hardware unable to deal with signal/power fluctuations that better components could handle just fine, AND that higher quality components would fix outright so as not to wear down components further in.
The point about support is important, above. Home builders should only buy components like the Asus "pro" series, that have warranties of their own. Not that you want to use them, but that you CAN use them makes the engineers pay closer attention and the company invest in failsafes.