The restrictiveness is the main point; using a grid is a relatively easy way to divide a page up into areas, and to make sure that things line up --- which in turn contributes to having an overall "clean look". (If things on the page look like they could be lined up but aren't quite, people read that as "sloppy", even if the things in question don't have much to do with each other functionally or topically. Using a grid is an easy way to avoid this sort of thing.)
i have real difficulty getting them to stick to any kind of structure.
a grid seems very restrictive to me, so assume designers would get pretty shitty.