[css-variables] var() for non-custom properties

Is there some reason that var() can not be used for non-custom properties?
Except shorthand, it would be a good idea.
We're already using a similar mechanism such as var() for non-custom 
property; currentcolor. Here is an example using var() instead of 
currentcolor:
..parent {
      color: green;
}
..child {
      /* background-color: currentcolor; */
      background-color: var(color);
}