It never lost 3 levels of casting, and really shouldn't. You remove 3 levels of casting and it starts looking bad compared to Mystic Theurge, assuming you replace the 9th level spells with 8th level spells. If you level them down it still means trading 9th level arcane spells for 9th level divine spells (normally a trade down), and means removing Miracle or getting into the funny state of using an 8th level slot for any 8th level cleric spell, and it still comes off pretty bad compared to other sorcerer options or just playing a Favored Soul (which is worse than Sorc). You can also compare to Rainbow Servant. If you go text then a fixed list cast 10/Rainbow Servant 10 is rather ridiculous. But if you go by the table (where they lose 3 casting levels) they're mostly reasonable (compared to what Sorcerers and Wizards can do) and strictly superior.