Basically I am kinda confused on how you attack interchangeably. Maybe you make one attack with offhand for every attack with the main hand and take -2 with all of them?
But then your full attack would look sth like this +18/+18+13/+13/+8/+8/+3/+3/-2/-2.
2 things to remember:
1. Only a single extra attack.
2. Interchangeability means equilibrium to both weapons (each offhand attack is just as solid as the primary attack that preceded it).
So basically, if you're to add but a single extra attack to the count (I never said you get an extra attack for each primary attack), you get a total of 6 attacks, so the sequence stops at the second +8.
Now take Quan, a 20th level Monk wielding 2 "small" [FONT="]non-piercing[/FONT] weapons:
(Edit: the "non-piercing" part is irrelevant on a regular attack (disregard))
Putting aside FoB and special cases for a moment, he'd be entitled to one of the following sequences:
+15/+10/+5/+15
or
+13/+13/+8/+8
It's not too difficult to see how [+15/+15/+15/+10/+5] is way better.
This would also explain why monks prefer even quicker weapon combos (ideal pairs / a "tiny" offhand weapon / unarmed extra attacks).
Ideally Quan would have the following basic sequence: +15/+11/+7/+3.
FoB would make it: +15/+15/+15/+11/+7/+3.
Now, I'm sure you'd agree that it's better than: +13/+13/+9/+9/+5. Or +15/+11/+7/+3/+15.
This compensates for the Monk's med BAB and almost matches the Warrior's offensive power.
Ole' Bob, wielding the same pair as Quan (having a basic sequence of +20 / +16 / +12 / +8 / +4 / +0), would now have: +18/+18/+14/+14/+10/+10/+6, which is obviously better, but against opponents with high DR, he might prefer a slower weapon that carries a lot more impact (with solid modifiers (STR, magicaal plusses, magical enhancement, W.Focus tree etc) +20 / +14 / +8 / +2 could easily turn out to be +37/+31/+25/+19, or +31/+31/+25/+19/+15 with Slashing Flurry - and that's of the top of my head with a 2-min thought).
And what do we get for all this effort to redefine TWF and weapon speeds:
1. Far better balance between the martial classes.
2. Far better balance between different fighting styles.
3. Saving precious feats from going to waste on the ridiculously long official TWF tree just to keep up.
(and a lot of other stuff).