MP3.com says that DRAM reduces battery life for certain portable MP3 players. Snippet:
Those who belong to subscription services such as Napster or Rhapsody have it worse. Music rented from these services arrive in the WMA DRM 10 format, and it takes extra processing power to ensure that the licenses making the tracks work are still valid and match up to the device itself. Heavy DRM not only slows down an MP3 player but also sucks the very life out of them. Take, for instance, the critically acclaimed Creative Zen Vision:M, with a rated battery life of up to 14 hours for audio and 4 hours for video. CNET tested it at nearly 16 hours, with MP3s--impressive indeed. Upon playing back only WMA subscription tracks, the Vision:M scored at just more than 12 hours. That's a loss of almost 4 hours, and you haven't even turned the backlight on yet.