I think people are largely misunderstanding the 2DS. $40 could make a huge difference for some people. The trailer even said that the focus of the 2DS was 'value'. It makes no sense to say that you can just turn the 3D off because if you already own a 3DS there's really no reason to get a 2DS (save for collection purposes as Syd has so kindly pointed out).
So basically, the 2DS is just an item for those who don't already own a 3DS and can't afford one, and also a product which will resonate pretty well with fans and collectors.
If there's anything I've learned from Nintendo it's that you should never underestimate them. Everything they do does have a purpose albeit hidden at times, and they always, always have a plan.
EDIT: Also think about it. The 2DS is basically a simple way-around high value. Nintendo can't afford to offer further price-cuts on the 3DS because they'll more than likely be at a loss (if they aren't already, because as you may know, basic game marketing standards scratch upon the concept that sometimes, you have to withstand losses in hardware in order to gain in software). It probably costs a lot to produce autostereoscopic 3D capable screens, so with the 2DS, scrapping that feature along with using only one screen (
[Link]) make it easier to offer a reduced price for a product that is otherwise similar in every way to the original. Nintendo really knows what they're doing.
(Ok I'll admit it looks kinda weird but whatever).
EDIT 2: Sorry haha. So as has already been pointed out somewhere above, Nintendo is releasing this alongside X & Y which is honestly quite genius if you think about it. How many Pokemon fans are out there? How many are dying to play X &/or Y? And now think about how many don't own a 3DS. Yeah the number don't really match up. So the 2DS is also a great venture in the way that it will allow people who can't afford a normal 3DS to buy one and play X & Y, and it would also make more sense to get a 2DS if the only reason you're getting it is for Pokemon (which is something I see more than often among some Pokemon fans).