I have the latest build of Web Essentials 2013 (1.4.1). On the official page of Web Essentials they say that in 2013 version fixed that issue:
1.4 - December 1, 2013 LESS/CoffeeScript Various bug fixes and perf improvements
1.3 - November 16, 2013 LESS compiler now runs in NodeJS Bootstrap class name validator
1.2 - November 4, 2013 LESS relative paths fix Various fixes and tweaks
but when I change and save imported file in Visual Studio, it does not update main CSS file anymore. It needs to save the host less file to make changes in CSS.
There is a screenshot of my options:
