To the end user using a touch less keyboard/mouse driven PC there is no need to upgrade to Windows 8. If they are buying new hardware, it will most likely come with Windows 8 preinstalled and they'll be most happy avoiding the Metro interface and defaulting to the Windows 8 desktop environment. Install a start menu replacement and most consumers will be happy as can be. It is with touch enabled devices such as tablets, multi-interface laptops, and PCs, such as the HP touch Smart series mentioned below, were Windows 8 and the new Metro interface really work.
I recently upgraded to Windows 8 on an HP 310 Touch Smart all-in-one PC and put it through its paces as a kitchen/family communication centric PC. Once the PC was set up I put the wireless keyboard and mouse away and depended solely on the touch screen and Windows 8 Metro interface. With apps like Netflix, All-Recipes, and the various news apps available in the Windows market the computer is a welcome addition to our kitchen. The ability to split the screen between two concurrent apps such as a recipe for cooking while watching a video streaming on Netflix was an awesome experience. Granted you won't be updating an Excel spreadsheet or writing a lengthy e-mail with this PC but then again that's not its intended or primary use. Prior to the upgrade it had become a fancy mostly forgotten gadget doomed to collect dust. When running Windows 7 overlaid with the HP touch Smart interface, software or the HP touch interface itself would often crash or revert back to the Windows 7 desktop default. It then was almost impossible to work with without getting out the mouse and keyboard once again.
In my opinion a touch enabled PC coupled perhaps with a tablet running the Metro interface is where Microsoft's new Windows 8 operating system will really shine and become a seamless part of many households. For Windows 8 to really sell, consumers need to be educated on the three primary uses touch, non-touch, and the hybrid experience.
An unwelcome side effect, fingerprints on none touch PCs! Once one works with the metro touch interface it becomes natural to want to do the same on all PCs. My advice, keep a micro cloth handy :-)