> having all functionality tested results in better docs

Yes, but its a tradeoff with execution time as well as maintenance time. Its one thing to write down all features in one massive session, and quite another to keep this list accurate over the next couple of releases. Thats easier with "living documentation" where a test fails as soon as a UI breaks, but the more unimplemented tests we queue up the harder it gets to keep under control. So start with the highest value ones, and combine smartly :)