Anyone can build a PC and get it working like lego as you say, but tweaking it for optimum performance with minimal gating takes skill.
I've seen many systems set-up with wrong RAM freq's, CPU's without thermal grease, bent or crushed CPU coolers and fans installed backwards. They all turned on, but didn't work worth a damn under load. The big one was a guy complaining his external HD wasn't working and I found he had forced the HD USB plug into his network jack.
Whilst I agree to a point, a sensible person would also do a bit of research, and get advice from experienced builders - plenty of overclocking forums out there with super friendly and helpful people.
It really depends on what your time is worth, and if it is better for you to have someone else build it.
Z...