Other DS models
Enter the DS
Nintendo DS (2004): Many people forget that this system did not start off as the juggernaut it later became. The DS sported two screens (the lower one was a touch screen) and a clamshell design to protect them, but it was fat and unwieldy, and far too big to carry comfortably in a pocket.
DS Lite (2006): Although the screens were the same size, the device itself was considerably smaller and thinner, fitting easily into a pocket rather than a backpack. Released around the same time as "New Super Mario Bros.," a system-seller for the DS, the redesign catapulted the system to widespread success.
DSi (2008): This revision sported larger screens as well as two digital cameras. Perhaps the most important part of the system was the inclusion of the DSi Shop: an online store where users could purchase exclusive software. This was Nintendo's first foray into an online shop for handheld systems.
DSi XL (2009): The system was functionally identical to the DSi, but sported 4.2-inch screens (the DSi's screens clocked in at 3.25). The system was generally too big to carry in a pocket, and Nintendo scaled the size back down when the 3DS launched in 2011.
Nintendo 3DS (2011): This was a wholly new system that kept the twin screens of the DS, but implemented 3D graphics that did not require special glasses as well.
3DS XL (2012): The 3DS got its own XL version a year later, which sported a 5-inch screen on top and a 4-inch screen on the bottom, but was otherwise unchanged.
Nintendo redesigning its handhelds is a time-honored practice that goes back almost two decades. You may not need the 2DS, but chances are Nintendo will design and redesign another handheld soon enough. Depending on which version you get, you may find the constant innovation either indispensable or ill-timed.