Well think about it like this - keeping the grass means we're technically feeding COWS with that grass. And since cow farts have been partially blamed for global warming, we're making it worse either way! Also, lawn mowers run on gas if I remember correctly (I haven't used a Lawn Mower in maybe 7 years), so that's another problem, unless you have a more old school mower.
Lawn mowers should be used for one purpose and one purpose only - chopping down zombies! (Dead Alive/Braindead. What's better than that?)