SteveC said:
One thing to be very careful about is your line of reasoning here. Power Attack is only useful in a campaign with creatures with low armor class. As soon as you get into a fight with opponents with a decent AC, power attacking has either a very small effect, or it has a negative effect.
In a damage-over-time test, the -2 penalty for 2WF is typically compared to a -2 penalty for Power Attacking two-handed (for +4 damage).
So, take an 18 Str guy just investing one feat in 2WF and doing longsword + shortsword, and compare that to him using a greatsword. He's getting the following damage output:
1d8 + 4 = 8.5 average w/longsword
1d6 + 2 = 5.5 average w/shortsword
2d6 + 6 + 4 = 17 average w/greatsword
The greatsword's total exceeds both the longsword and shortsword put together. However, this is mitigated by having two chances to damage a target versus only one (which is what the "2WF suxorz" folks generally don't take into account).
Now, let's throw in EWF (waraxe) and Oversized 2WF:
1d10 + 4 = 9.5 average per swing w/greataxe in primary hand.
1d10 + 2 = 7.5 average per swing w/greataxe off-hand.
For the cost of three feats (or two for a dwarf), the total damage output matches the greatsword's, while having two chances to inflict damage rather than one tilts the scales (as SteveC points out, we're already in a situation where AC is effectively boosted by taking a -2 penalty).
Now, there are situations where the two-handed weapon still has a big advantage, like when a character has to move 10 feet or more to attack. That's where feats like Two-Weapon Pounce (PHBII) and Dual Strike (CV) can clean up.
One odd thing about Two's rationale is that he says it's somehow different for rogues than for fighters. The same "delivery system" strategy that a 2WF uses with his sneak attack damage also applies to other forms of bonus damage, such as Weapon Specialization or Favored Enemy, magical weapon bonuses and so forth.