It's hard to get a feel for the actual numbers as far as who uses what and the people in the commuter forum aren't necessarily representative of all commuters. There's a strong contingent that feel racks are pretty much mandatory for a bike to be a worthy commuter though personally I don't agree with that sentiment.

In a way it doesn't really matter. There are pros and cons to each and just because either backpacks or panniers may be the most popular choice for a given group, it doesn't mean that they'd be the best option for a given individual.