I have never had the experience of using rice and have it as fermentable as cane sugar or dextrose. Both those sugars are refined, the sugars from the rice and corn conversions can still leave you with dextrins. So the the added sugar is capable of being completely metabolized by the yeast, the rice/corn not so much due to starches that have not been completely broken down into simple sugars. I use adjuncts in beer and it produces a lighter flavor with out being super dry or overly attenuated. I have made Belgian beers with 20% rice and they are no where near as attenuated as when 20% sugar is used.
I have found rice adds a touch of sweetness to the beer as well. That's my take on it. I may be way off, but that has been my experience. I personally don't like sugar in an adjunct beer, rice and corn add to the character of the beer, IMHO.