Mark, there's a number of possible reasons for that guideline. First, by having only one layout ("drawing") per file that allows multiple users to be thrown at a job. Second, drawing tracking systems usually track by filename not by drawing; with a 1:1 relationship between file and drawing it minimizes management and storage issues. That also comes down to requesting an existing drawing for modifications, its not beneficial to sign out the entire 20-drawing set considering that other projects which need some of the other 19 drawings will have to wait until the entire set is checked back in.