Actually... it's for every 1 lb of footwear, it's like carrying an extra 6.4 lb of weight on your back. [Legg SJ, Mahanty A. Energy cost of backpacking in heavy boots. Ergonomics.1986 Mar;29(3):433-8.]

I have been almost exclusively using trail runners for the last 9 years and couldn't be happier. I only consider boots in deep winter when I need a really stiff sole to kick steps, going cross country when I am expecting lots of sharp pointy vegetation that would rip nylon apart, or when I am doing trail work and want to be sure that if I drop my axe it doesn't take off my toes.

I have been fine going cross country in most landscape, in scree fields, hoping over dead fall, etc. A light pack with light shoes is a lot more recoverable without hurting yourself. Misstep... no biggy... midflight slightly shifting of your weight and hop over or do whatever you need to do.

--mark