Here's where it was stated that a Wizard 10/Cleric 10 is unplayable. I'm glad someone does agree with me on this, since so much of this thread is negativity towards multiclass spellcasting. While taking levels in other classes may cause a wizard, or cleric, druid, etc. to lose out on caster levels, I think the point of multiclassing into something else is to provide flexibility. Besides, if you were a multiclass cleric/wizard, why on earth would you not take the Mystic Theurge (I think it's called. In the DMG) PrC to continue advancing both classes' caster levels? You'd be a 20th level character who could cast spells as both a 17th level wizard and cleric.