You're exactly right on those results Moritz, but a low level balanced group needs the spell support from that peon wizard. Like Mount, Message or Comprehend Languages. Cast at the right moment a cantrip like Daze can be deadly to a fourth level fighter.
Same with the high level groups, some monsters can't be taken out by spells alone, they need to be pummeled by cold steel.
Wizards start getting more powerful at higher levels, and that happens at around ninth level.