I think the one thing that is confusing about the Borderer example is that they get the Feat at 5th level, irregardless of meeting the prerequisite. Therefore, at 5th level they have a BAB of +5, and their off-hand attack would be at +5.
The Feat states that they get an additional attack, but the way it is worded, it seems to imply that these additional off-hand attacks match any additional Base Attacks they gain as they level; when they gain 6th level, they are now +6/+1, and with the Feat, have off-hand attacks of +6/+1.
So why give them the Feat at 5th level if it is of no benefit to them at that level?
Yeah; its pretty strange. My guess is that its probably a mistake.
Regardless of how you rule it, a 6th level Borderer will have +6/+1 main hand, +6/+1 off-hand. Only thing thats unclear is what happens at 5th level. Does he get:
A) +5 main hand, +5 off-hand
B) +5 main hand, +5/+0 off-hand
From a strict
reading of the rules, I would actually say that option B above is correct; the description of I2WC just says that you gain an extra off-hand attack at a -5 penalty, and it mentions BAB only in regards to what happens at higher levels (BAB +11, and beyond). It seems a bit odd that you would get more attacks with your off-hand than your main hand weapon, though, and because of that, option A might be more logical.
Since one of my players will most likely level up and become a 5th level Borderer during tomorrows game, I should really make up my mind on this one. :? You folks have any more input?