I think people should read whatever the hell they feel like reading.
I probably read "adult" books most of the time, but I also love the Harry Potter books and thoroughly enjoyed Hunger Games. Haven't read Twilight, and never will - but I don't necessarily hold it against people for liking these kind of things.
I think you need a mix. I love reading books that challenge me, or make me think, or present complex, rich worlds. Sometimes, though, I just want a fun ride.