While I do think that doing throw-away prints of webpages is bit questionable, there is still some value in making webpage print out nicely. I think "printing" PDFs is one of the easiest robust ways of archiving web pages. It creates nice static self-contained local copy of the content, which is readable basically on any platform.

Another example would be e-readers (eg Kindle) which in ways are closer to print than web as a medium. They are not currently really equipped to handle web-pages well; Send to Kindle extension seems to do some sort of readability-like heuristic when formatting web-pages to be sent for Kindle. It would be kinda nice if print css would be good enough to work as-is on Kindle (etc).

Of course there is the question of the overall quality of current web-design. Many pages are just ridiculously busy to begin with, something that somehow becomes accentuated when printed out. Maybe designers should spend a moment after building a print layout reflecting if all that stuff that needed chopping away for print was actually necessary to have in the first place.