While building a PC is very cost effective, I agree that it does not make sense in a corporate setting. That's why you have companies like Dell for where if there is a problem, you get a replacement the next day.

For me personally, I would only build a pc for myself while everyone else get Dells. If I come across a problem on my PC, I can easily fix it for myself. I can't imagine working at a shop with all custom PCs. It would be a nightmare.