This is really an age old debate. We are not the first to discuss the issue of Mac vs. PC. If you would like to join the existing debate regarding PC vs. Mac,
clique here.
Their marketing people know who their target audience is. Just because you buy a whole system at once does not meant that there are other people out there who do not need to replace only certain components of their computers.
My mom is a perfect example of Apple's target audience. She has a fairly decent Mac setup, but her Cube (hard drive) is a bit outdated. Every other part of her system is in fine working condition. Since she works for a state agency, they tend to attempt to save money where they can. This new hard drive only option is perfect for someone in her position.
Everyone's situation is unique. Don't assume that since you have your own experience that your neighbor will ahve the same experience.