Where the Idiomatic style guide is good for smaller sites, single-line CSS has its place. If you are using some of the newer design methods and have a lot of either atoms, molecules, organelles, or modules, submodules, and utilities, single-line allows multi-member teams to find rules more easy.

This is why larger companies such as EA (Sims 3 Store) and Yahoo are switching some of their sites back to single line. I work in a team of developers. Even inside a SASS/LESS environment we find it quicker to maintain our code.

Sure, we have our modules and quarks broken down into separate files, but even then a multi-line file is too much to scroll through. Because we are following Nicole Sullivan's rule on separating the structure from the skin, our atoms and modules have a limited number of properties to vertically scroll through (3 or 4).

There are some exceptions of course, like Inuit's famous beauton class.