for example kingdom hearts for ds

If that game is Super Smash Brothers Melee then yeah.

Depends on the game:

upcoming games that might qualify:

Brawl
FF XIII
MGS
Ninja Gaiden 2
Mario Kart Wii
The World Ends With You
Dragon Quest IX

You can add whichever ones suit your taste. If KH is it for you, then oh yeah, but let me tell you this, it's not smart to sell your DS right now, considering that its lineup this year is the first "golden year" lineup, and looks to have some of the best RPG's on this, or any system, so far this gen.

Just imo, that is.

Depends on the game, and how much you love the game. I bought the N64 for one game. For almost a year, Super Mario was all I had for the Nes. Street Fighter was all some gamers ever needed for the Snes. And Tetris Attack is the only Snes game my mom has.

long answer short: yes.

A system is only worth keeping so long as you plan on using it again. It doesn't matter if its for 1 game, or 20.

Well said. That is the correct answer put in a very simply and easy to read format.

 Yeah. Uh, not really. You may find in twenty years you regret very deeply getting rid of something. Of course, it depends on how much of a packrat you are and how much storage space you have. There are many considerations.

 Of course, this is a completely wrong answer, as we, or at least most of us, are human being which are not just moved by logic and thinking, but also by emotions, feelings.

As for myself, I don't even consider a system is worth getting for one game, except if I plan to play at least 3 years of it. 

You never know what other games might be announced that you'll like. I'd keep it.

I guess if you're not interested in FF4 and FF6 remakes plus the rumored MGS DS game.