Giggleton, your proposal is interesting. However, it would require some kind of verification of the "play time" on the side of the user. I was thinking of a predefined real-time window starting from the moment of purchase because in this way the content provider can be sure that the user can't circumvent the verification mechanism :-)
Of course, setting a general, fixed, real-time limit of 24 hours creates its own set of problems. For instance, I was forced to introduce an exemption for short-lifespan content (those which are usually played a single time, and have a duration well below 24 hours).
Your idea is certainly more sensible in principle, but I have doubts on its technical feasibility (read: prevention of workarounds).For instance, for movies you could be given a maximum of 15 minutes of playing time to decide if you like it or not; only if you stop playing within that limit, you can switch the movie for another one.