I'm pretty sure it does not only drain batteries, but also hinders performance (which is why reviewers kept praising the fact that Mario Kart 7 was able to run at 60fps when playing online with your 3D turned on). I wouldn't mind if they released a few games where the 3D is disabled and you get better graphics, but when it comes to games like Mario it's a really sweet addition. I'm pretty sure Nintendo won't do that though.
As for sales influence, I really don't how how it affects/has affected the sales. On one side there's the "OMG mom, the 3DS can show 3D images without any glasses YOU HAVE TO BUY IT!", and on another side there's the "Watching too many 3D images makes you nauseous!"-rumor which could have stopped caring parents (who are unaware of the parental settings...) from buying it.







