Sorry, I gotta rant a bit about this.
Imbalance? The DM runs the game! I don't need rule imbalance to kill of the party if I so choose. Nowhere does it say in any of the books that I MUST use CRs to determine what monsters to throw at the party so if I throw a Great Wyrm Red Dragon at the party then that act is totally supported by the rules and I didn't use any "imbalance" kill them off. I'm so fed up with this "Players vs. DM" mentality. I get tired of players who shoot holes in a DM's plot because it doesn't abide by "THE RULES" and then complain about the mediocre story. You all know the type and they typically metagame. They'll analyze what level spellcasters are by the number of dice you roll for a Magic Missile attack or Fireball and then keep track of how much damage said caster has taken. The instant the caster has taken more hit point damage than their "calculation" has allowed for then they'll "helpfully" point it out to the DM and the rest of the players. If the DM doesn't take care of it to their advantage or at least spit out a reason for it then they'll get ticked off and sulk for the rest of the game. I view the game as a communal storytelling session. The players and the DM all play to tell a story. If the DM takes advantage of a rule to make the story interesting then so be it. I'd certainly hope that everyone's DM doesn't actively try to play against the Players.
Sorry for the rant. I'm just sick and tired of that mentality.