There are DVDs that lock the control to prevent the owner from skipping over the ads and trailers at the beginning of a DVD he’s purchased.
DVD De-Crypt will help with that. For the ‘doze platform.
Also, if you run Linux, you can rip the DVD and edit the Menu file to play just the movie. Then you can simply delete the ad-ver-tise-ments and unnecessary trailers and whatnots.
It does not make it all that much a smaller file, once you consider the size of a typical DVD, but the ability to run just the movie all by itself is such a relief the next time you wish to watch the movie! :)