A waste of time. Whatever cardio does, weight training accomplishes much better. (And if somebody asks what I do for heart, I swear I'll shove a barbell up their ass if I ever see them).
Besides, I feel like a douchebag running in one spot or riding a bike that goes nowhere.
You want cardio? Play a game of hoops or handball of something.
I should mention that I live in NYC so I walk and bike for real most every day.