The following is an excerpt from page 6 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding the Skirmishers and ranks.

Although skirmishers in close combat do group up into a formation resembling ranks and files, they never count as having a rank bonus. Despite this, a formed up skirmishing unit does have flanks and a rear, which will count for combat resolution bonuses. This means that, once in combat, a skirmishing unit can become as much of a hindrance as a help if there are enemy units that can subsequently charge them.