The problem I see is. With each built in software within the DS the price to make them is more expensive. I would rather have more memory to store Linkplay games than another built in program to the DS.
Nintendogs isn't neccessarily a simple low memory product anyway. It could be a very advanced game that deserves an entire cart to design.
The chat program alone will help appeal to women. Nintendogs would be the perfect launch game for girls to buy.
Besides women. Children will love the game. (And I know that we as older gamers hate the idea, but Nintendo appealing to children is still smart.)
Little girls will love to play with dogs on the go and have virtual pets that act and behave like real pets. Its literally the next evolution of the virtual pet. You can interact with them physicall and actually play real games with the animals instead of just push buttons.