I've just realized why it's important to have such a high chance of failing a spell and the reason is that every class should focus on their respective skills. Playing a priest then means being able not only to cast spells but also to hit with almost 100% probability. Otherwise, due to the fact that spells do quite some damage, every class would be the same, spamming lightning bolts all the time.
Just went from lvl49 priest with "dark mass" one-hitting whole room to lvl1 thief with 20% chance to hit a bat with "open wounds" and no wisdom skill so no chance to increase.
It's certainly nice to have to play the thief as a thief, but i wonder how it will work out since he can't even repair his own equipment and is bound to get hit all the time.