In Favor of the Oxford Comma
When writing a list*, we typically separate each item with a comma. Before the very last item comes 'and' (sometimes 'or'). You can include a comma before the 'and', but you don't have to. This particular comma, which would come immediately after the second-to-last item on the list, is call the Oxford Comma, and its necessity is a matter of debate among scholars. But the general consensus is that you can choose to use it or not, as long as you're consistent.
I find that more and more people these days don't use it. I suppose that if you could choose to do slightly less work when writing something, you'll go with that option. However, I find that in some cases lists may be confusing when the Oxford Comma is omitted.
One problem arises when 'and's or 'or's are a part of the list itself. Consider this list of music genres: (1) jazz; (2) country; (3) rhythm and blues; (4) rock and roll. Now suppose we put them together in a sentence and don't use the Oxford Comma. We'll try a few different configurations. Which ones do you find easy to read, and which are a little confusing?
Sam likes jazz, country, rhythm and blues and rock and roll music.
Sam likes jazz, rhythm and blues, country and rock and roll music.
Sam likes rhythm and blues, jazz, rock and roll and country music.
Sam likes rhythm and blues, rock and roll, jazz and country music.
Frankly, I find all four of those confusing. It's hard too see what the music styles actually are, or which combinations are real ones. Is 'rhythm and blues and rock' a style, or 'blues and rock and roll', or 'country and rock', or 'roll and country', or 'jazz and country'? Are 'jazz, rhythm and blues' three separate genres? Since we're familiar with the actual styles, it's not too hard to figure it out, but we shouldn't have to figure it out; if it was well-written, we wouldn't need even a second to decipher it.
Commas also help us know when to pause. Try reading those sentences, and pause when there's a comma (but don't pause where there is no comma). Go ahead. Did that sound strange to you? Especially that first sentence? Or did you automatically pause between music styles? In that case, we should have a comma there.
Let's try those sentences again, but this time, expect to see a comma separating every item in the list, even the last two items:
Sam likes jazz, country, rhythm and blues, and rock and roll music.
Sam likes jazz, rhythm and blues, country, and rock and roll music.
Sam likes rhythm and blues, jazz, rock and roll, and country music.
Sam likes rhythm and blues, rock and roll, jazz, and country music.
Does that read better for you? Do your eyes naturally separate the genres at the appropriate places? When you read it out loud, pausing where you see commas, does it sound better? For me, the answer is 'yes' to all of these questions. I knew exactly what the distinctions are, because the commas define them; where there is not comma, I knew not to separate the genres, because I expected one otherwise.
Another way in which the lack of an Oxford Comma can be confusing is when some or all of the items in the list contain a lot of words. Consider this sentence with a list of three verb phrases:
Next summer, I'm going to fly to Rome with my family, eat and drink lots of the local cuisine and visit as many sights as I can.
Here's that same sentence, this time with the Oxford Comma:
Next summer, I'm going to fly to Rome with my family, eat and drink lots of the local cuisine, and visit as many sights as I can.
It's not difficult to understand the first sentence, but the second sentence is certainly easier, with no room for confusion.
Even if you want to argue that the Oxford Comma is not necessary, I'd argue that it's helpful. Anything that improves readability is a plus in my book.
* a list is three or more bits of information in a row, in which they are all the same part-of-speech.