We're not talking about the same thing.
On a desktop computer, it's ok to have ports in the back since that's where you plug in things you don't usually unplug (printer, iPod dock...) Mac keyboards have had USB ports on them for a while, and that's where you plug in often-removed peripherals.
On a laptop computer, you are most likely plugging and unplugging things all the time - it's a portable computer, duh. This is where having to reach in the back all the time becomes annoying.
Also, it's perfectly possible to use iMac's ports "blindly", or with a very slight peak behind the screen. Now imagine how much of a "peak" you'd have to take in order to insert or remove a disc from that mocked-up design.