I don't bother taking any straight 'mages' because direct damage spells suck in W8 and the alchemist & Bishop handle everything else better.
A bishop is does not become as strong as a mage in combat until much later. It's always lagging behind in development 1-2 levels if not more because it doesn't get 25% bonus, needs more experience, and usually it focuses on other schools of magic at the same time. You will need to cheat to make it better in combat.
The alchemist on the other hand is very good, but it doesn't have all the spells that you would like in some situations or progress slower in some areas. That's why I like both the mage and the alchemist together.
I don't think direct damage spells is necessary to survive or anything, but in my experience they do reliable damage and often more damage, because the monsters are often nicely grouped up together. It's also satisfying to watch them all go down in one round when you know that a "brute force approach" would take much longer.
Spellcasters may not be as powerful as fighters later in the game, but I think they dominate the beginning-middle part of the game.