The "closed box" argument is frequently raised as a criticism of Apple computers, however the PC industry is pretty much following that trend. Take a look at the "all in one" desktops from Sony, HP, Dell, Acer. None of which feature much in the way of expansion slots. Laptops, by their nature are "closed boxes" so there can be no argument that Apple's are unique in that situation.

Perhaps PC fans are remembering the days of the original Mac or Mac Classic, but even then, remember that those came with built in networking, SCSI ports, support for PostScript . . . all stuff that had to be added to nearly all PCs at the time.

For high end users who need modular construction, Apple does have a device for that.