Depends on what you need. In the general case, No. If however you need something 'special', like the very fastest CPUs, lots of RAM and/or multiple GPUs for example, then Dell/HP etc. tend to greatly overcharge for these configurations.

This has changed somewhat with the decline of Moore's Law. Companies are keeping equipment longer, past the point that OEMs typically provide warranties or even parts.

When you know that's going to happen you want to start with something that uses standard parts rather than vendor-specific ones.

For the small or midsize companies though that need that extra power, building can work if you have the talent in house. Cheaper, more standard, and more powerful parts and everything is very reliable these days so its no longer a major support issue.