Where I work, we used to build our own PCs. We were able to do it relatively cheaply, and they were normally better spec than PCs we bought it.
The trouble is, where I work, the company likes to buy things in because that gives us all sorts of protection in the event something goes wrong (warranties, legal rights etc), and, in the event something really goes badly wrong, someone we can sue.
That and the fact that we are increasingly buying Macs, and we have to obey the terms of any software licences we use, so Hackintoshs are out (the OSX licence forbids running OSX on hardware that is not Apple branded).
Regarding what Dell are doing, I am not defending them because I think what they are doing is at best wrong and at worst, a con, but they aren't the first computer company to do this. I read a story while doing my degree that to upgrade the storage space on one ICL mainframe, you needed to call an Engineer. All the Engineer did was flick a switch and activate the read heads on the other side of the discs.. Another example of how the computer industry can sometimes scam people. Where I work now, we used to have a lot of Sun workstations. To replace the CDRom in these usually cost around £500 to £600. These were generic CDRoms with a different firmware and slightly different interface (IIRC). Had we been able to use normal CDRoms, they would have cost us about £150-£200 at the time.