The student council president promised longer recess, higher grades and ice cream and cake. That would be much better written as "longer recess, higher grades, and ice cream and cake." That's a silly example, but using the final comma in a series before "and" just makes this more clear. So, why not?
Today a story broke about a court case where a student who learned NOT to use the serial comma wrote a piece of regulation that is causing all sorts of legal mayhem (and one company a lot of money).
Here's the story: