During the Mac-on-Intel transition, Apple introduced PC-notebook-style hibernation feature into its MacBooks and MacBook Pros.
Hibernation itself isn’t bad. Apple’s defaults are. Joe Kissell does a good job summarizing the badness.
SmartSleep is a System Preferences pane that does the nasty work of making Safe Sleep work like it should.
I recommend the settings in the above pic: smart sleep enabled, 20% battery level. That should keep you relatively safe without having to blast your hard drive with your machine’s full RAM contents on each and every close of the lid.