@Crumbly Writer
As unnecessary pauses tend to throw readers, requiring them to double back and reread passages.
And when I don't see the serial comma, I stop and double back. I reread to understand the sentence — to make sure the last two items aren't associated. I do it every time.
I like Grammar Girl's logic:
Although the serial comma isn't always necessary, I favor it because often it does add clarity, and I believe in having a simple, consistent style, instead of trying to decide whether you need something on a case-by-case basis. I also think using the serial comma makes even simple lists easier to read. Really, unless space is incredibly expensive, I can't imagine why anyone would decide the best method is sometimes leave it out and sometimes add it in.