Not sure how one can debate that having almost an entire class' worth of abilities for three levels isn't broken.
There are plenty of ways to do that without giving up a
single level's worth of abilities. They include: Fusion, Shapechange, Gate and all of the other binding/summoning spells, the Leadership feat, Thrallherd, etc.
Are those broken? Many would argue yes. I think they are almost certainly more "broken" than this, as this has much stricter limitations than most of those things.
One possible solution to this "problem" would be to allow unlimited advancement of the Mindstealer drone's hit dice, and cap the level of abilities that he can steal at his
class levels of Mindstealer drone. IIRC, the original monster worked like this, didn't it?
Of course, adding levels devoid of class features isn't a very attractive idea, but if you actually add meaningful class features, this thing may wind up being even more "overpowered" than the current version. So I think the current one is actually better than this "fix".
Forget about what you're getting for "just three levels". Look at the character as a whole. Is having the abilities of Class X-3 and Class Y-1 stronger, weaker, or equal to having the abilities of either just Class X or Class Y? The Mystic Theurge comparison was spot on. Even using "cheesy" early entry methods like Precocious Apprentice, a Wizard 1/Cleric 3/Mystic Theurge 5 is still arguably not more powerful than just Wizard or Cleric 9. The latter both get 4th level spells, while the first doesn't.
Mixing in things like Tome of Battle, Ardent, and Ur-Priest, which multiclass better than most classes can help tip things in the direction of the drone/mystic theurge, but those are specific examples, and even then likely aren't much (if any) ahead of a single-classed, specialized build like Wizard 5/wizard PrC.