Just computing average damage for the rogue posted for ease of comparison. I'm trusting the numbers provided are accurate, and not going through the build line by line. Though I am curious, why use daggers instead of short swords? Having d6s for weapon damage instead of d4s would make it easier to condense stuff, and been marginally more effective assuming everything else still works with it.
Going with an enemy that has an AC of 40, because that's a decent middle of the road level 20 AC.
Attack routine with haste is: +44/+44/+44/+39/+39/+34/+34/+29
1d4+13d6+45 (average: 93, crit: 143.5) on 5 attacks, and 1d4+13d6+43 (average: 91, crit: 139.5) on 3 attacks.
Main Hand
Spoiler
Attack 1, 2, and 3:
10%: Crit Threat, 95% confirm (143.5), 5% not (93), average: 140.975
85%: Hit. Average: 93
5%: Miss. Average: 0
Average Damage per Attack: 93.1475
Attack 4:
10% Crit Threat, 75% confirm, 25% not, Average: 130.875
65% Hit. Average: 93
25% Miss. Average: 0
Average Damage per Attack: 73.538
Attack 5:
10% Crit Threat, 50% confirm, 50% not, Average: 118.25
40% Hit. Average: 93
50% Miss. Average: 0
Average Damage Per Attack: 49.025
Total Mainhand average damage: 402.0055
Offhand:
Total offhand damage: 210.87
Total damage with two weapon fighting:
612.8755
Now as a point of reference, two weapon fighting has a -2 penalty to hit, so without it, the last two main hand attacks would be altered as follows:
Total Damage gained from losing the -2 penalty: 19.3145.
This means that for two weapon fighting to not be viable, you need to find 3 other feats that combine to adding 191.5555 damage to the rogue's full attack.
Seems like a pretty tough sell to me.