just my 0.02 cents here. why it's a must? nothing really fancy for the usb 3.
your printer doesnt need usb3, your monitor doesnt need usb 3, mouse, no, keyboard no.... external drive? well, sata 6G is plenty enough and faster than usb 3, and cheaper and more reliable, and Mac OSX has bulit-in SATA driver, piece in mind for not dealing with any 3rd party faulty driver. your scanner doesnt need usb3, webcam, no. I know there are more professionals need faster speed but the pro guys would not use iMac but MacPro right?