In at least three senses, it's a silly question: no matter how much of a type A work addict you might be there will be time enough in your life to read at least two different works of fiction, obviously we each read a great deal of factual material every day without lamenting its lack of moral lessons, and for youth almost any reading is probably better than no reading.
But still, what's bothering me is that I can't figure out even one worthwhile moral lesson that can be pulled out of the Twilight series at all.
In the Harry Potter series, school was literally a magical place which basically all students looked forward to attending. Furthermore learning mattered, in the real sense that by studying and paying attention in class you could learn to do practical things that you weren't able to do before. Benefits accrued from staying loyal to friends who merited your loyalty, and although there were situations involving self-sacrifice they were ones which were both unavoidable and in which the prospective benefit far outweighed the cost.
But in Twilight, what? Bella attracted all this supernatural romantic attention through no cultivated virtue or personal effort on her part, she didn't see much value in school since presumably at least one of the two available monsters would scoop her up for a hopefully effortless domestic life, for the same reason she didn't seem to see much value in maintaining human friendships at her school, and she apparently attempted suicide to attract the attention of her paramour despite being on good terms with her parents and hence presumably having an inkling of the far greater pain her suicide would cause.
Perhaps I'm going out on a limb here, but I think it's especially unfortunate that the Twilight series was still in heavy cultural rotation during the Royal Wedding; I'm sure Kate Middleton is bright, but neither her intelligence nor any other personal trait other than being attractive and thin seemed to be cited by the world media in explaining Prince William's proposal. So, in either case the bleak message which seems to come across is that attracting a spouse is all that matters in life, but there's nothing you can do to make this happen other than starving yourself and hoping.