the directors/actors commentary can be very good on some disc's and i have (sad person that i am) watched most of my disc's while listening to it second time around :-)
the alternative endings on some disc's can sometimes be good and a couple of good music videos, outtakes, deleted scenes, making of or games for the kids thrown in can be a bonus.
depends on the disc i guess. if its a quality movie i really like and comes in two disc format with all the bells and whistles i will pay more for a nice box and extras and have a whole evenings entertainment instead of a few hours. i dont think it spoils anything as i know its not real to start with. i think its a good thing. its nice to see some releases now give you a choice of just movie or two disc special editions so if you dont want the extras then just buy the movie.