Some carbon bikes have eyelets for mounting racks. I'm pretty sure the Madone doesn't. It's a racing bike.

That said I have two bikes. The one I race on and the other one. I don't race much but I do use the "race" bike for commuting most of the year. It doesn't have clearance for studded tires so I don't use it in the winter.

If a Madone was my race bike, I'd commute on it too. There are people in my building that commute using Cervelos. I can keep my bike in the office though. If I had to lock it up outside all day I probably use a cheaper road bike.