Reading man-pages does do a lot for you. And in my experience, I don't spend a lot of time while on the same engagement/project/whatever writing shell code across BSD/solaris/GNU-linux/etc.
And honestly, if you can write a sophisticated sed substitution for BSD without reading the man page, is there really THAT big of a barrier lying between you and writing the same thing in GNU/Linux, or non-POSIX-compliant sed, or whatever else? You read the man page for 10 seconds and find the flag that means what you want to use. If it's different, you explore.
This is totally not a rant at you, and I sincerely hope you don't take it that way - I just haven't found myself utterly foiled by the differences between say, RHEL v SUSE v BSD recently. There are differences, but hell - there are differences between versions of languages, between terminal emulators (scripts that work in bash but not in zsh and vice versa), etc.
This may have been true(r) twenty or thirty years ago, but in a contemporary Unix installation, it is not true at all.
The set of tools is often NOT small (there are thousands of them); they are NOT simple (two examples: 1. the ls man page and the huge number of options it takes; 2. shell quoting rules); they are NOT standardized (I regularly run into incompatibilities between BSD-derived and GNU tools, for example, the -E option for grep); and they are NOT sharp.
In summary, the whole "small tools that one thing only, but that do that one thing well" thing is a stupid, outdated mantra that may have made more sense many years ago, but not anymore. For some reason, people keep on blindly believing it even though it bears no relation to the reality of modern Unix programming.