I switched to smaller vanes for the sole purpose of having better fletching clearance, and I couldn't be happier. My arrows fly like bullets and I haven't noticed any loss of stability or accuracy whatsoever. FYI I'm shooting the 2.25" QuikSpins and they fly great out to 40 yards, which is the futher I've shot with them thus far.

The only other advantage I can really see is the decrease in weight and drag during flight, which in theory should increase your arrow speed a bit. My Allegiance shoots plenty fast as it is, so this wasn't really a factor in my decision to change fletching size.