A majority of open source projects have a paucity of documentation. However, they still have value as a working system. I argue that improving the documentation of a codebase intended for general use is one of the highest value activities that a developer can do. Although documentation may not be directly contributing to the codebase, the writer provides a valuable service by understanding the code and imparting that knowledge to others.
I would go so far as to say that there should be open source technical writers. They look for interesting projects to write basic documentation for. While there are a smattering of tutorials for certain frameworks, they are not condensed, are not authoritative, and are often misleading because of changes to the framework or convoluted examples. When the core developers move at the speed of the forum or even IRC, it's tough for a new person to have anywhere near that amount of knowledge. A centralized wiki with pertinent and up-to-date examples is quite useful.