Komamura got suckerpunched halfway across town. Iba's standing right next to the guy who did it. Why the hell wouldn't he fight?
Anyways, the difference between Shikai and Bankai Ikkaku is stupidly large. It's been shown more than once that the Bankai's where most of his strength lies. He's not like Kira or one of the others, with some fancy ability he can rely on in the meantime.
That said, good chapter. I am both pissed that Ikkaku's manly pride made him lose, and happy that Iba gets to something other than look cool.
Also, yay Yumichika flipping out.