The 3DS is not "a slightly fancier DS." That's like saying the Gamecube was a slightly fancier N64.
Even without the 3D stuff, the 3DS is far more powerful than the regular DS, and that contributes a great deal to the lower battery life.
Stop focusing on the gimmicks of the hardware and using them as an excuse to ignore everything else about the system. It makes you sound like a Sony fanboy circa November 2006 trying to defend his $600 console purchase.