@homunculus:- I think....every games has its rules, if you play the game and break the rules, it means cheating.
If the rules is well define and BALANCE. It will be a good game and everybody loves to play. Example:- The chess games. or maybe sports games.
as for the % based shooting for Xcom-1, I believe it is one of the BEST BALANCE rules that enhance game play experience. It guaranty that you only able to fire certain shoot, or do certain action in one turn. And as the game advance further... it still maintain the rules. (you can only fire 1 aim shoots).