One of the commonly heard gripes about multiclassing spellcaster classes is that their caster level gets screwed. A couple of levels of fighter or rogue means you're 1 spell level behind, and you're dealing (or healing) less damage, your buffups have reduced duration, etc. This gets worse the more non-caster levels you take.
What would happen if you used _character_ level instead of _class_ level for the purpose of determining level-dependent effects of spells? Thus, for example, a level 6/6 fighter/cleric would still be limited to 3rd level cleric spells, but would cast them as a 12th level caster. Ditto a 6/6 rogue/sorcerer, or whatever. A 6/6 cleric/wizard would have access to 3rd level cleric and 3rd level wizard spells, but would cast them all as if 12th level.
What would happen if you used _character_ level instead of _class_ level for the purpose of determining level-dependent effects of spells? Thus, for example, a level 6/6 fighter/cleric would still be limited to 3rd level cleric spells, but would cast them as a 12th level caster. Ditto a 6/6 rogue/sorcerer, or whatever. A 6/6 cleric/wizard would have access to 3rd level cleric and 3rd level wizard spells, but would cast them all as if 12th level.