Since the goal was not to have too much mana at the beginning of the battle, but rather not to make it more difficult for heroes using magic to cast spells.
It’s really annoying when people say that knowledge doesn’t matter because it doesn’t kill, unlike SP. Knowledge should matter and will matter, in full force, just as much as any other characteristic, not only in PvE but also in PvP. No knowledge – no access to the best magic, sorry :)
For example, all humans have 10% knowledge, so it’s difficult for them to cast a single “Enslave” spell for 30 mana even once per battle (because it requires at least 6 knowledge), which is why there are no level 5 guilds in the city. But they can cast spells of levels 1-3 without any problems.