Each community has its own requirements and those of the Chef cookbook authors in general are different than those of Ruby library authors.
Lets say that I right a cookbook for apache2 on Ubuntu. What should I name the gem? Many other people will write this cookbook, particularly since mine only works for Ubuntu. Perhaps 'chef-apache2-ubuntu-btm'? Where do you store such metadata so a user can find, use, and contribute to a cookbook?
This isn't a simple solution and is a bit unique, which is why Opscode has been crafting a solution at http://community.opscode.com. It is easy to get a copy of a cookbook from the repository using 'knife cookbook site install COOKBOOK_NAME' and you can search the site through the web interface and using 'knife cookbook site search' to find the cookbook you're looking for.
The community site development has been slow, but continuous. In December we added source browsing so you can review cookbooks without downloading them.
http://www.opscode.com/blog/2011/12/14/chef-community-site-c...
There's still much to do, but I think we're (I work for Opscode) headed in the right direction for the project and community.
That said, perhaps none of that really matters; I wonder if all this comes down to hidden assumptions re: constituencies and heterogeneous skillsets, which I think is what you were pointing at in the beginning of your comment. i.e. "you need to learn the Chef toolchain" may be a more sale-able/scalable message than "you need to learn the Ruby/gem toolchain".
There are many views out there for what the answer should be for Chef cookbook workflow. What you are "baffled" by, another person uses daily and argues for.
When using knife to download a cookbook into your git managed chef repository, it creates a 'vendor' branch to store the upstream version. You can utilize this to create diffs and patches to contribute back to that cookbook project. It is up to the author exactly how to do that.
Some really like git submodules, others prefer one cookbook per git repository, some just have one company-wide repository and ask you to fork and apply branches against.
I think you're right about the 'Chef toolchain.' There are [large enterprise] users of Chef who don't care that Chef is written in Ruby nor particularly want to learn Ruby. They want to use Chef. Opscode continues to strive to provide tools to make the workflow easier for Chef users as a whole over preference to a particular toolchain, especially one tied to and solving problems for a single language.