The "Top40" commercial music listeners are actually quite a narrow demographic. They simply have disposible income, are easily marketed to... Actual overall music sales dwarf the proportion of those who rush out to grab the latest britney CD, however when tastes become more fractured and diverse they become more challenging targets for unified marketeering. Hence our media is saturated by mass cultural icons that comprise the largest slab of unified consumable mindshare.
Liking a form of music that is not 'commercial' (wrong word really, unless you got given it for free, all music is commercial) does not really make us unique, or even in a niche market. It simpy means we belong to pop-culture rather than mass culture. Pop-culture generally represents slightly oppositional discourses than the dominant mass culture. However, mass-culture often 'incorporates' elements of pop-culture to better capitalise on it. A good example is the jeans phenomenon. Remember the 80's when blue jeans were just the shit? Everybody wore them. Soon some rebels started to wear ripped jeans around as a fashion rejection of the mainstream. This got popular. So... Mainstream jeans manufacturers started selling, you guessed it, pre ripped jeans.
Somewhat like the rave scene. In a curious parallell, Just Jeans now sells phatpants. Of course that isn't what they call them, but they look exactly like fucking phat pants, and the advertisement has a ferry corsten backing track... See what is happening? Phatpants used to be something you sewed together with some cheap-ass fluffy shit you bought from lincraft, or purchased at some creepy elitist chapel st. scungehole like strange days. No more!
Commercial/mainstream is constantly in flux. Its one big inherent contradiction.