Despite x86 compatibility, a large part of the value of Apple laptops comes from the fact that they are able to easily run OS X. It would be extremely difficult to sell an Apple laptop that did not come with the operating system preinstalled, since most of the potential buyers would expect to be able to run OS X. Replacing the hard drive would not help in this circumstance.
also, with unibody macbook (pro)'s it's trivial to replace HDD/SDD.