I'm a little late to this thread, but there's one thing that remains to be said. Maybe two.

As far as necessary can't live without, you're already set. Everything mentioned here is useful and advisable, but in my opinion not necessary for a daytime commute.

However, I will make an addition if using a cell-phone is not a convenient answer to mechanical problems: a multi-tool and a frame pump. I formerly would have considered the frame-pump as an optional accessory but twice in the last year my CO2 has failed me, so I've moved that to the necessary column.