Having two screens is pointless.
I believe two screens for the DS and future handheld systems is a Good Thing, and here are a few reasons why.
I believe two screens for the DS and future handheld systems is a Good Thing, and here are a few reasons why.
1. Differentiation
DS was initially made to throw a cog into the PSP launch. At least initially, though, Nintendo talked of it as a third pillar, but for it to appear as something more than just a more advanced GBA, and to provide something different from PSP, it couldn't just be a regular portable game machine of the same basic type as PlayStation Portable, Game Boy Advance, or even the 1989 Game Boy. The second screen certainly differentiates it from all of those.
As far as considering why a second screen would still be a decent idea for future systems, though, this is not a factor.
2. Touch screen as extension of control pad
The touch screen can be used as an extremely adaptable extension to the regular face buttons, while the main game view continues on the top screen. Jump Super Stars and Metroid Prime Hunters both do this in very different ways.
3. Split into levels, like storage
Computer systems have different levels of RAM for best efficiency. For a simple example, GameCube has 24 MB of fast memory, and an additional 16 MB of relatively shitty but cheap memory. Even a system that doesn't have an obvious split in main memory like that is going to have a small amount of cache memory on the processor, and possibly a disk drive which can keep a lot of data but is slow compared to main memory.
Think of DS's setup in the same way. If it was one big regular screen, there'd be no touch. If it was one big touch screen, it'd either be more expensive or not as big as both DS screens combined to hit the same price. 256x192 touch screen + 256x192 non-touch screen, versus maybe... single 320x240 touch screen.
4. Logical design decision
Presented as a discussion between two Nintendo R&D; guys, Ricardo and Keizo.
Ricardo: So we need something... different.
Keizo: Touch screen!
Ricardo: OK. What sort of case design? GBA SP's is pretty popular.
Keizo: Yeah, but I don't think serious frequent prodding is going to work so well on a flip-up portion. The touch screen should be somewhere sturdier.
Ricardo: OK, so more original Game Boy Advance.
Keizo: Ehh... it also might not be such a hot idea to have the screen out in the open like that. Yu Sixpack can scratch up his GBA screen with his car keys, but if it's easy to mess up the touch screen like that we're screwed.
Ricardo: Well, shit.
Keizo: Yeah.
Ricardo: How about... wide and clamshell?
Keizo: Seems a bit of a waste to have a flip-up portion and not use it for anything, though.
Ricardo: Speakers, K. Speakers.
Keizo: Yeah. Nintendo innovates again with handheld gaming's first 4 inch speaker!
Ricardo: Shut up, Keizo! How about... another screen? As long as it's not a touch screen it's not exactly going to shoot up the cost of production. They can stick a map on it. Whatever.
Keizo: Good enough for me.