Got a friend who watched me trimming a text block once and asked why I was ruining a perfectly good book. Had to explain the whole process of squaring up edges before casing in. Sometimes it's easier to just show them the finished product.
My old shop teacher used to say you can't explain a square spine, you have to show one. I keep a few before-and-after photos on my phone for exactly this, @sanchez.kevin. Someone sees a book with a wobbly text block and thinks it's fine, then they hold the same book after it's been backed and lined. The change in how it opens is what clicks for them. They finally get that the work wasn't about cutting pages, it was about making the thing last.