> Furthermore, if you write your own sentence, and finish with quote of an entire sentence, why isn't there a period for both sentences? Brian said, "let's go.".
You'll never completely avoid ambiguities no matter what you do. Take the question mark in the following sentence:
Brian asked, "should we go?"
Is Brian asking a question or the person who's quoting Brian?
Right, the point being that with certain styles of writing it makes a lot more sense to resolve for aesthetics rather than a simpler grammar. This perspective was largely ignored by the Slate article.
The question sentence could be resolved "logically" by placing the period outside the quotes, and possibly dropping the question mark entirely as you are already describing the quotation as a question. But if you're telling a story, including a question mark and putting it inside the quotes and leaving off extraneous periods is the best way to convey the overall meaning.
You'll never completely avoid ambiguities no matter what you do. Take the question mark in the following sentence:
Brian asked, "should we go?"
Is Brian asking a question or the person who's quoting Brian?