Play some old games before you buy new ones?

Given that we already seem to know when the Steam Summer Sale will go live, it just makes sense that you should play and finish some of your back catalog first. That seems to be what Valve was thinking when they kicked off their Spring Cleaning Event today. This event rewards you for playing old games that have been sitting in your library, games you haven't played yet, and even some new, free games they're offering for the duration of this...