Possibly. Sixth level spells are considered level appropriate abilities for an eleventh level character, ninth level are level appropriate for a seventeenth level character.What you are suggesting would mean that any caster with more than a handful of monster levels would be weaker than any single classed caster of the same level. And no, single classed spellcaster isn't our balance point, it's just that we want these classes to be useful in as many campaigns as possible, and cutting all spellcasting monsters out of any game with optimization going on isn't good.