Apple locks OS X onto specific hardware because that's the hardware it gets tested the most on. This means that OS X works as intended on this hardware. It ensures a certain level of user experience, all the time, across the board. This is why they have such a sterling repuation and this is why so many of us choose Macs. If there was no lock and if any old company produced a PC that ran OS X, Apple would have less control over the hardware being used and OS X might not work as intended on it. This would pose a serious problem for the average user (not to mention Apple) which comprises the bulk of Apple's market. Meaning, we'll stray into Windows territory and the ocean of problems that come with it, which we as Apple users are trying to avoid in the first place!
Okay, you are aware that there's absolutley nothing special about the hardware Apple uses since it went Intel right? Take the Dell Mini 9 as a good example here, that installs using a RETAIL copy of OS X and needs just a single driver pack to get everything working as required. So by your logic if us humble users can get hold of hardware that's OS X compatible with the same driver set as Apple uses.... what exactly would be the problem?
Oh, and while OS X has a very good reputation for stability Apple's hardware rep in recent times has taken a battering (expanding batteries, failing GPU's, dodgy screens, poor quality cases, exploding Magsafe's, faulty iPod screens etc etc etc) so this whole thing about Apple's hardware being somehow superior to anyone else is just rubbish. Design is good, reliability not so much.