Pumping the ability that is used for damage (usually strength) is a noble endeavor.
However, as you said, +2 to ability only yields 1 damage (1.5 for THF, or 0.5 for offhand)
This means, for two handed fighting, main ability score is more important than for TWF.
Getting direct bonuses to damage seems to be worth more.
For example, let's say you have a character using the heroic claw set (
http://ddowiki.com/page/Category:Vulkoor%27s_Might_items), +3 artifact to hit and damage; +5 str.
For a TWF build, you'd need a +11 or +13 strength item to replace the lost damage from claw set.
For a THF build, a +9 strength item would almost match it.
However, if you're using tactics, then pumping your main stat has an additional benefit.
Extra damage dice (flaming, etc.) add up. One thing to remember is that on Reaper difficulty, these get rounded down (often to 0). So big numbers are worth more than small numbers.