In recent times, many series have opted to forgo the full-length opening theme song/sequence and replace it with either a sort of half song (think Gossip Girl and Greek) or just a flash of the show's name (think Lost and as of last season, One Tree Hill). Even shows that used to have full length opening credits have recently started to abbreviate them - like Desperate Housewives and Grey's Anatomy.
I agree that some shows don't need a full opening sequence (I can't imagine any sort of opening credits that would do Lost justice), but in general, it's a great way to set the stage for the show.
10 of my personal favorite opening credits/sequences that I believe are particularly defining to their respective shows:
What are your favorite TV theme songs/opening credits?