Not any more, except in two cases -
- You want a gamer's rig or similar targetted machine (or you want to build it around one or more exceptional components)
- You wish to save cash by recycling components from your existing machine.
For my own machines, I tend to build myself due to the first factor, but not everyone lives in that niche.
Ever since I started building computers, they've been steadily progressing along the product line to commodities - competing on price rather than any other factor. Alienware resisted that trend, but even they are now introducing a budget line.
Due to the steady drop in component pricing, they're now hitting the stage of true commodities - one breaks, you toss it out and buy a new one. In that environment, mom and pop shops will have a tough time competing with the big boys, who get volume discounts on both hardware and software. The operating system was a rather high percentage of the total cost of the last system I built.
These guys are going to have to shift focus from system builds to services. The only way I see for them to make it is to be willing to disinfect virus-laden boxes, salvage or transfer data, troubleshoot networks, etc. - and to do it all on-site.