Buying magic doesn't make sense
Aight, at first - most things related to magic don't make much sense.
Let's take a look at the Grukk-Back Spearshooter which costs a whooping 4020 Magic. Even if you exchange like once per hour (500 Magic) at the mine which still gives a reasonable amount for the waiting time and let's assume you never miss it and are never too late you could build like a maximum of 3 per day (If you sleep at night make that TWO). So what is the sense of the hero skill to upgrade the training speed of these?
And yeah - you COULD exchange emeralds for magic instantly. The point of this exchange would be to make players spend emeralds - which is a brilliant thought. Problem here is that you have to pay 160 emeralds to exchange 4000 fish and 4000 mushrooms to 4000 magic instantly. At the black market you can just buy a Grukk-Back Spearshooter for 110 emeralds (That is WITHOUT any units discount!!!), so you would need to be incredibly stupid to use the instant exchange feature. I REALLY doubt that anyone ever used it and if there is someone who did it they must be a true fool.