It would be great if every site offered a printer-friendly view. The web would be so much more usable. But I also believe the ultimate authority on how a page looks and prints should be you. What if company policies force designers to include ink-draining advertisements on the printer-friendly version (cough cnn.com cough)? What if you don't like the font a page uses, or only want to print part of the page, or want to merge multiple pages into one print job? A printer-friendly stylesheet won't do that for you, but PrintWhatYouLike will.
I totally agree that PrintWhatYouLike will mostly be used to get around poorly designed sites, but print stylesheets are only part of the solution. The other part of the solutions is giving you the ability to take a designers page and customize it to suit your own unique needs.