Converting for Strength Potions would be nearly impossible to balance. Either you can convert for 10+ of them at level 1 and kill nearly any boss in a single hit, or you set their cost at 300+ CP and the race becomes nearly unusable.
It's not really a matter of "purely mathematical equation," either, unless you know ahead of time exactly how many CP you will have and when. Which is rarely the case outside of very specific, likely to be removed before final release, strategies. (As an aside, Elf/Gnome and Dwarf/Halfling also have a "purely mathematical" superior in just about any situation, but again you'll rarely know all the variables ahead of time. It's just more notable with Humans/Orcs because increasing damage output is more likely to allow for one of the aforementioned very specific strategies, as happened with the Strength Potion and Crusader bugs.)