Your carpet will love you.
When you want your car to have that glistening appearance, you add a coat of wax. For your indoor environment, your carpet (or at least your significant other) would appreciate not getting hosed with bunch of sweat from your Spinerval or Carmichael training DVD workout. Save the trouble of both cleaning the carpet and making amends with that special someone and put this piece of wonderful plastic on the ground. I have one that I use for my training mat when I need to during the winter. When it is not being used as a sweat collector, I use it under my bike and Ultimate Pro (found here on Backcountry) repair stand. Those flecks of black grease have no where to go with this mat under your bike.