Well, you can only pay the full price for all the mastery scrolls required on one particular champion - so obviously the more masteries/scrolls you already have on that champion, the less interesting it becomes to pay 800 gems to unlock all of them. But as you say, by far the longest part of scroll grinding is for the red scrolls, so if you only just started with the reds, you can still save a lot of grinding by paying the 800 gems. 

It is cheaper to just do the grind and buy refills with gems, though.