Suppose you and some of your friends form an amateur Shakespearian troupe. You give plays on Sunday afternoons in your neighborhood park. It's a hobby, it's enjoyable.
Suppose an "entrepreneur" happens to watch you one sunny day and approaches you about performing in his theater for a paying audience. He sells tickets. He gives you part of the money. Suddenly, you have moved from engaging in a hobby to "productive labor" in capitalist terms -- that is, labor which produces surplus value.
You're doing exactly the same thing. It has exactly the same use-value to your (now paying) audience. Only the intervention of the capitalist has changed its nature in capitalist terms. (As an aside, it will probably now become less enjoyable to you as well.)
As Rakesh points out, the criticism that this category of "productive" labor doesn't include all labor which has social value (housework, childrearing, etc.) is a criticism of capitalism, not of Marx.
Tom Condit