There is not only no reason a preprocessor like SASS can't someday efficiently process selectors like he describes, but theoretically it would be easier with something like SASS. In SASS you could keep the color declaration alongside the other CSS declarations, then the preprocessor could simply find all the declarations with that variable and string them together with commas.
Unfortunately the SASS preprocessor doesn't do that... yet. The main draw of SASS is the ease of development and maintainability, not reducing the byte size of your CSS files. Personally, I don't want to spend the time manually hunting through CSS to combine selectors in order to save a few bytes.
To give you a more practical example, instead of a color like #efac68, imagine the variable held the value 1em or 10px. If you wanted to increase the width of the gutters between columns, it's easier to change $gutter than to try a find and replace of 10px in your CSS, while trying to only change the values you want.
Unfortunately the SASS preprocessor doesn't do that... yet. The main draw of SASS is the ease of development and maintainability, not reducing the byte size of your CSS files. Personally, I don't want to spend the time manually hunting through CSS to combine selectors in order to save a few bytes.
To give you a more practical example, instead of a color like #efac68, imagine the variable held the value 1em or 10px. If you wanted to increase the width of the gutters between columns, it's easier to change $gutter than to try a find and replace of 10px in your CSS, while trying to only change the values you want.