I just posted a thread in the touring forum about the same subject - namely using a single seat toddler trailer instead of a backpack or racks & panniers.

Long story short, I got a smokin deal on a single seat, high quality toddler trailer ($45 brand new in box, down from $179). I DO plan to use it with my toddler, but I also bike commute 5 days a week in fair weather, and want to do some touring this summer. I used to carry a backpack, which was fine for my 7 mile commute, but it was not very comfortable and made me VERY sweaty (not ideal before work). Also, capacity is limited with a backpack, and it makes me very top heavy which significantly changes the feel of my bike.

The trailer holds 50 lbs, has well built 16" wheels and an aluminum frame, has a rain cover, and is light enough. It attaches by a simple lug through the rear QR skewer, fits all of my bikes, and installs in seconds, without tools. What I really like is that the volume is plenty enough for work clothes, tools/flat kit, lunch/groceries, and whatever else I might pick up during the day. Also, it means I can ride my #1 road bike without a backpack, frame/saddle bags, or racks/panniers, and just unhook one cotter pin to drop the trailer for spirited intervals during my lunch hour

The issue I'm having mentally is that I NEVER see anyone doing this . . . why?! I am part of two very large cycling groups in town and I do regular group rides with professional types like myself who still find time to get plenty of miles. NOBODY I know, even the ones with kids, uses a converted toddler trailer for this purpose, although plenty of people I know still use backpacks or racks/panniers. Why?!

It seems like a great idea, but it worries me that my hardcore cyclist friends haven't "caught on." Or perhaps they have, and there is some huge downside that I'm missing?!