comma

It is always best to have consistency when writing, so I always use the serial comma.  Though sometimes a sentence is easily understood without it, as in your first example, there are other times that can get confusing. If you only use it some of the time, the reader cannot always be sure what you mean. 

I am going to lunch with John, an attorney and a musician.  This indicates that you are having lunch with one person who is both an attorney and a musician.

I am going to lunch with John, an attorney, and a musician.  You will have lunch with three different people.   

Yes, in a word. Some guides will say this comma, called the 'Oxford' or 'Harvard' comma, is optional, but most prefer its use.