We see the occasional spinning side kick in MMA, so it does exist in some form. I think most posters have it right, though, in regards to throwing a regular side kick with the back leg...it's too slow. Heck, you don't see it thrown that often in TKD sparring for the same reason.
I have thought, however, that side kicks thrown with the front leg could be quick enough. Certainly not as powerful as the rear leg, but they should be fairly quick, and have a nice line to strike the opponent's ribs/body. Probably best thrown off a short hop/skip to get a little more power. I'd imagine the first time or two someone saw you throw it, they'd expect a front-leg roundhouse and left their leg to check it, which might not catch the side kick. Could be a nice surprise.