Unitless or not unitless?

Matt Smith:

You should always specify line-height as a unitless number (say this into the mirror five times). That way descendent elements that specify a different font size will inherit that number instead of a fixed line height.

Counterpoint in the comments by David Khourshid:

Unitless line heights are fine, but aren't the only way to use it. If I want an exact, perfect vertical rhythm implemented in my stylesheets, I would absolutely use a line-height *with a unit* …

Unitless or not unitless? is a post from CSS-Tricks