The goal of wizards in PE is, ultimately, flexibility. If you need/want someone to help the melee dudes, a properly-grimoire'd wizard can do it. If you want to cause a bunch of AoE damage to soften up a pack of scrubs, a properly-grimoire'd wizard can do it. If you want to target a specific defense or a specific damage resistance with a certain type of attack, a properly-grimoire'd wizard can do it. But if you want to go toe-to-toe with an equal-level fighter in melee, you're going to die. If you want to trade damage with an equal-level ranger, you're going to die. Wizards have more potential abilities (i.e., spells) available to them than any other class (casters included), but their access is always limited by their equipped grimoire. Switching grimoires disables spellcasting for a short while so it's really not something you want to do in combat unless you're desperate.