Part of the popularity of hopping up the little bike is because in addition to a learner's bike, it is also popular with the vertically challenged. Unless you want a two-stroke, there aren't many "real" bikes available with seat heights of less than 36". The XRs are kinda heavy and dated, their only advantage over the TTR being the full-size front wheel on the 200. So let's say a gal who's 5'2" wants to learn to ride, and pretty soon she's wanting more performance, but doesn't want to sell it and buy a different bike because the next step up is a big one--either a 125 2-stroke or the 250F. Hence the aftermarket industry steps in. Also, a lot of racetracks are introducing mini bike classes with cc limits of 150.