I have nothing against any other reporting tool , however I feel here you should have explained the users with SSAS or any other tool will give some magical solution with absolutely zero effort in reporting end then it will ultimately turn out not to be the case.
With Webi with very minimal reporting effort this can be achieved using calculation context variables..
Also there is no need to build a cube just for this requirement all that is really needed is to educate the power users how BO/Webi handles these measures..