For apple it is a matter of Quality control. If they control the hardware & software they can reduce development costs since they only have to support a finite amount of software unlike windows that has to support way to much hardware. I still like the challenge of getting OS X on other platforms. Nothing beats a Hackintosh netbook.
In college I had a great computer by Power Computing. It was the first company licensed by Apple to make apple clones for the brief period they tried it.
It was great! So much faster than anything Apple had out at the time. Yeah. I wish they didn't stop that program.
I made a hackintosh dell once. It was great, but I wouldn't do it again.
For me personally, it's not worth the hassle and the risk of incompatibility. Reminds me of when I used to build my own desktops from parts. It used to be cheaper that way.