I think it depends on what you hope to accomplish during that time. Like a PP said, if you decide to do Soarin and FEA, that will probably eat up your entire time. But if you plan to use it to ride a few things you didn't have time for, like Spaceship Earth or Imagination, and do some shopping and enjoy more time at EP without as many people, then its definitely worth it. We try to use our FP for the hard stuff, then ride the easier to get stuff during the afternoon, and possibly with an additional FP, and then save the easiest to ride stuff for EMH, or things we want to ride again (my kids cant get enough Figment) and doing our shopping during EMH. We also try to wander around the WS a little bit during that time. While the crowds wander out after Illuminations, we like to take in the night there, its got a great vibe. Basically, it just depends on what you hope to get done.