This theory is one I always find so frustrating. It's amazing the amount of information you can pick up from a good fiction book - and I have no specific data to prove this but I believe it is easier to retain such information.
Obviously fiction does include made up stuff; but some of the best fiction is actually well researched. Some examples:
- Dan Brown novels are full of real history, geography and so forth.
- Swiss Family Robinson has lots of natural history (one of my favourite books actually)
- Tom Clancy novels will teach you an awful lot about the military and military hardware etc.
It might not be specialist information, but it's amazing what you can learn from fiction :) Avoiding fiction because you can't learn anything from it is, well, a bit short sighted.