Well..for one the sugar will dry onto the weed and when sugar, C6 H12 06 (idk how to do subscripts..) burns it releases the hydrogen and oxygen, leaving you with carbon, this nasty black tar looking shit.
Try it, cook some sugar straight in a frying pan, see what happens, you do not want shit like that in your lungs.
Just do what everyone else is saying, buy some flavoring meant for shit like this, or simply buy better tasting weed...