I always thought that switching between justified and unjustified text just left justified the lines over inserting space to justify them, but other than that the line lengths remained the same and so the overall length of the book and page breaking remained the same. I was so wrong.
By inserting a generous rightskip the line breaker doesn't try nearly so hard to get as much on a line as it would if it were trying to reduce the amount of space interspersed along the line and a paragraph can easily grow by 3-4 lines as a result. My 91 page book went up to 93 pages and most paragraphs got longer and so all my beautiful efforts to get paragraphs balanced (in my diglot) and pages broken nicely was for nought.
Unfortunately, I can find no way around this problem. Setting a rightskip increases the background skip that the line breaker uses and so changes the choice of where to break lines. If anyone has any wisdom to share, I would be interested in hearing it.
