It depends on what you mean by "shouldn't". The Oxford comma is optional and considered syntactically correct if used. If you mean that it is more concise to leave it out, then thats another issue.

http://en.wikipedia.org/wiki/Serial_comma

EDIT: Ignore this. I thought he was referring to the first line of the post. "Follow the principles below to write sentences that are clear, to the point, and easier to read."