You mean like backwards compatibility? It's always a smart marketing decision to include that in systems, if only for the fact that you'll be butchered in the media for not including it. Backwards compatibility mainly caters to two types of people. Those who missed the previous generation(For example, I didn't buy a Gamecube, but I can still buy GC games for my Wii), or those who sort of don't want to be alienated by being told that they need to buy a new system that won't play any of the games they have now. I think it's only natural if you have 20 GBA games, that you'll want to be able to play them on your DS, I mean, who wants to carry two handhelds around? And for the others, a gigantic backlog of old games is always good for a young system that is still building up its own repertoire of games.
At the same time, I can see why it could be a negative factor. If you look at the Wii, a lot of games still support the GC controller, and then shoehorn the rest of the controller options on. A lot of Wii games have become GC games with 16:9 widescreen basically because of this, and don't really innovate beyond anything that came out last generation. This could possibly be the exception, and not the rule though.