Is it a good idea for businesses to build their own PCs, or are they better off buying pre-built systems from computer manufacturers?
With businesses, reliability and minimal downtime are usually key issues.
If you build in-house, you not only have the cost of the build, but of all the support issues. You'll wind up with a complete mish-mash of hardware and software configurations, and a nightmare of lack of consistency. You might be able to select a better blend of components, but at the price of having to potentially spend unknown amounts of time sorting out problems.
Personally, from a business perspective, I want to spend my time doing what I'm good at and what generates revenue. I don't want to be sucking down valuable staff resources building computers for a modest saving, if any at all, when that time could be spent doing what the business is supposed to be doing .... unless of course, you have lots of qualified staff sitting on their hands or twiddling their thumbs. And in that case, you have a bigger problem than who builds the computers.
Of course, all that doesn't necessarily mean buying from Dell, etc. You might be able to find a smaller, but still reliable, local company that will build to your spec and use quality parts, and provide service. Just be prepared to pay for it. A lot will depend on how many machines you're talking about. The issues involved over 5 or 10 are a bit different to those of 5,000 machines.