3 additional USB 2.0 ports including 2 on the side for easy access, a card reader, and a TV tuner. What I would give to have any of those things on my iMac.
So..... you would trade 24-inch 1920 x 1200 display, 2.8 Core Duo, ATI Radeon HD 2600, firewire 800, optical audio, 1TB HDD, and OS X for 3 additional USB 2 ports, a card reader, and a TV tuner?
They also see something in having form factors as well rather than forcing you to buy an all in one whether you want one or not.
Looking at their quarterly profits that looks like a winning strategy. If I were to place a bet my money would say looking at their quarterly profits is the reason why they are all offering an AIO.