You should check out what kind of plane you'll be on and then checkout the entertainment available on that type of plane with that airline. A lot of long flights have your own independent screens to watch as many movies as you want, tv shows, games, etc. With a laptop your only going to have 3 hours of battery life anyway.
On the other hand: Most of the planes equiped with personal screens also have personal power outlets so you can plug in your laptop. If you have games you want to bring along, work, your own movies, tv, etc. then it would be worth it.