I think is a really good question; I have used touring bikes for commuting (including a Trek 520) and am looking for something lighter. yet to me a "road" bike rarely seems to work - typically not enough brazeon's for fenders and racks, not enough clearance for larger tires w/fenders; etc.
A bike I would recommend you consider instead of the Trek 520 is the Jamis Aurora Elite or Salsa Casserroll. The Aurora Elite is marketed as a touring bike but the build is not quite as rugged as the Trek 520 (or Surly LHT), and it weighs about 23 pounds. The Salsa Casserroll is more of an all-rounder road bike, and maybe a pound less, with fenders inlcuded.
I think both bikes end up being several pounds lighter than the 520, which for a regular commuter is noticeable, IMHO.
I am personally considering both, have not pulled the trigger....I have a couple of other bikes on my radar that are a bit more $ as well (e.g., Gunnar Sport).