Purpose of the usage section 

I've reverted edits that came with the explanation "The purpose of this section is not to provide usage advice or to use intuitive but inaccurate descriptions like '"doer of the action.'"  Actually, the original text was well-supported by citations to usage manuals, and the particular words "doer of the action" are directly from The Merriam-Webster Dictionary of English Usage, which linguists consider the best usage guide.  The edited language did not cite any support for its assertions.

More broadly, even if the replacement language did have good support, I don't think it would be as good for this section.  I recognize that Wikipedia is not a how-to guide, but in a section on English usage, I think the text should explain its meaning in a way that will be understood by someone to whom it is a new subject.  Statements such as "the passive voice is used to place focus on the grammatical patient, rather than the agent," can be understood only by someone who is already knowledgeable about the subject.  That statement is also less specific than the text it replaced.