Skinny: Worth It?

I have worked out for the past 15years - if not daily - then close to it. I work out because:

(A) I like being able to play around with my kids
(B) I like feeling strong and master of my own body
(C) I like how I feel after a workout
   and
(D) I like to look good

My sister (who also stays fit) and I were discussing this and I wanted some opinions. Some studies say it is better to have a little extra weight on you. Some say that that might be true but not if it is on your middle. Some still say we women need to keep our BMI between 18-25.  Besides the Health Factor there is also the Public Opinion Factor which leads to my other question:  Are girls trying to be skinny because THEY think it makes them prettier than other girls or because guys REALLY PREFER thin girls to their curvier companions? 

I recently read a book called Hungry by Crystal Renn. She believes that every one's body has a set weight it wants to be at - a weight that is easy to maintain. She believes that this weight range is rather large and all places on it are equally healthy. I'm not exactly a fan of the book but I didn't necessarily hate it either.  

I know that I can eat less without feeling hungry and quit working out and only put on about 4 -6lbs (all on my arms, hips and thighs)  before my body hits a maintainable weight. I also know that if I do this my flexibility and strength go out the window and I feel more drained throughout the day. So my questions are these:

1. What are your thoughts on a healthy weight?
2. Are girls the cause of the skinny craze or are guys?
3. Is working out worth the time and effort or something to let go of?