Package booktabs encourages users not to use vertical rules. The purpose of this manual is to concisely and clearly describe the features provided by LaTeX, not make value judgements. (Personally, I find almost any table is best off with few or no rules, especially vrules, but that's irrelevant here.)
Concerning other packages, should they be mentioned (packages that enhance core functionality) or not mentioned at all. I have included references to a few packages here and there, such as beamer and geometry. When the practical reality is that a given package should almost always be used instead of/in addition to some core feature (slitex, [a4paper]), I think it is only helpful to say so. This is a manual describing core LaTeX2e, right? Right. And that is plenty. We are still nowhere close to having all of LaTeX2e covered. > Maybe it would be good to describe somewhere how to do thicker/thiner The purpose of this manual is to concisely and clearly describe features provided by LaTeX, not provide solutions to "how do I do X" for every typographical or technical X. In general, please don't add text "encouraging" or "discouraging" certain styles of output. Just be factual about what the features do. k