It all depends on what you are allowed in the army...
I know with most Western military you have downtime, and are allowed to amuse yourself however you wish exactly like getting home from your normal job - I have seen photos from friends of a group of them stationed on a ship who were playing as a band - all of them having personal musical instruments on board (the photo I am thinking of of them playing on the helipad during a clear time on ship).
I would do a bit of checking and see what you are allowed, if you are allowed a computer ten keeping it is an obvious choice - being able to surf the web, listen to music and watch dvds is always nice.
If you are not allowed it you have two choices. Either cut your losses, sell it now then upgrade when your tour is up - you will then have a current computer when you are out, and lose a lot less money by selling it when it is still a recent model, rather than 3 years old. The downside is that you won't have a computer - so you lose the use of it if you are on a break at home or any other opportunities that a computer would come in useful unless you have other access.
Option #2 is to just keep it, you don't have to faff about selling it and do have it available when it could be useful. The downside is that since you would own it longer, it would depreciate in value more...
Someone did point out a good idea above if you intend selling it - if the army works in blocks, then it may be quite likely that a lot of guys are leaving when you are arriving, and if they are not allowed the use of a personal computer, then some may be on the lookout for a new model for themselves...
Thanks for the reasonable response. Umm I think I will consider selling it before the apple care runs out. I really would like to keep the Alum Macbook cause it just looks gorgeous lol
So... I will have to sell it before Christmas I guess... Hopefully I won't be needing much of needs of Macbook when I am in the University during that time...
Thanks everyone for all your help! I really appreciate your efforts
p.s. I wish my Korean army is like the Western army lol I am pretty sure that personal Laptop won't be allowed since personal shampoo or skin lotion isn't even allowed in the Korean Army... Even if I sell the Macbook I really wish I will get into the American army base in Korea :C