For a commuter, fibre is almost useless. It's great for making a light racing bike, but will be a poor choice for a commuter. You will want a lot of accessories that a fibre bike simply will not accommodate. Things like fenders, racks, and strong stands. Then fibre is easily damaged where a good steel frame can take a beating. Sure you can make a fibre frame work, as you can any frame, but it's not going to be the best choice. Certain bikes are really meant for certain uses and are best left to those uses.
My current commuter is steel and 31+kg and yet I get to work in the same time as my old commuter that was under 10kg. Weight is not really that important in commuting.