For those asking why, color replacement is enough to have them implemented. Once...

Kilimanjaro on April 12, 2012 | parent | context | favorite | on: CSS Variables published as First Public Working Dr...

For those asking why, color replacement is enough to have them implemented. Once your stylesheet grows, you'll have your theme colors scattered all over the place. So having them in one place at the top and making minor changes to the theme is a great time saving feature.