Williams' own books are fully justified. She admits to being unhappy about the uneven word spacing, but likes the look of fully justified type, at least for some projects. It's a trade-off, she admits. If, as I've suggested, justification isn't all that important, it might not seem to matter if you're forced to use left justification. Documents set in columns often benefit from full justification.
Legal documents contain lots of lists.
Notice what's happening at the left margin of the bulleted list. This makes the left margin pulsate in and out. And immediately to the left of the list, the ragged right margin of the adjacent column is swaying to and fro. By some unlucky chance the opposite right margin might almost line up or the bulleted list be in the leftmost column. I find the erratic left-right shifting of the space between the columns distracting.