I would say that a trailer is right out for commuting. I've dealt with one on tour, and have used one for grocery shopping from time to time, but I think that dealing with it everyday on my commute (i'm thinking parking/storing the bike here, mostly) is just a pain in the rear.

A touring bike has the mount points for front and rear racks and fenders.

For daily commuting, a pannier mounted bag of some sort (briefcase or backpack types being the most ideal for my situation) is a necessity. I cannot stand wearing a backpack or having a messenger bag over my shoulder when I'm riding for more than a mile or so.