The thing is people "think" they out grow one bike and move up to the next. So their thought process is why start on the little bike when you'll out grow it in a year? So go straight for the big stuff.

Well let me tell you what. Small used bikes are easy to come by and much easier on the wallet for a beginner. When it's time to move up the little used bike is worth just as much as when you bought it(if you didn't wreck it) So if anything you only be out maybe a couple hundred bucks. Well worth the learning experience.

Your first bike isn't your last. (unless you give up riding)