[media-queries] Using calc()

Is there any reason not to allow the use of calc() in a media query?
It feels like having the ability to be precise about lengths would be a win.

For instance, if I have an element with a margin of 2cm and a width of
100px, I may want to specify:

@media (max-width: calc(4cm + 100px)) { … }

Right now, it is disallowed and implemented nowhere.