Quite right. But a good DM would not allow munchkins to break the game like that.
Exactly, DAO allows you to do it 
Arguably the examples I provided are mostly acceptable. You sacrifice something to gain something stronger. A Mindbender is only good for the telepathic powers and the classes that pick the prestige are usually bad at combat or affecting the world state. DMs tend to stop classes that are inherently overpowered. A lvl 20 wizard is able to completely destroy the world. Nothing more needed, no optimization, no extra knowledge. Just google the spell list and check the most devastating spells. Done.
Same case with overpowered classes in DA games. They completely break the balance of the game. Oneshotting the Arishok shouldn't happen. The only reason it happens is because of modifier stacking. Attributes is the obvious suspect here because Assassination spec makes cunning provide double the critical damage bonus. I have more examples, but by now I think you get my point ^^