Another good reason to shoot RAW + JPEG rather than just JPEG is that JPEGs are compressed files that uncompress when you view them and them recompress when they are closed. Like a spring, they eventually deteriorate over time. NEF (Raw) files do not. Also, if you have just taken the "shot of your life," and it is a JPEG, you will have less post processing control. The downside to RAW is the size of the file which must be stored on your memory card and then onto your hard drive. This, of course, can be remedied by deleting the RAW files that you don't like or need.
If you do have some JPEGS that you want to keep forever and you don't have a RAW file to go along with it, you can save the file as a TIFF before you open and close it again. TIFFs are even larger than RAWS, but like RAWs, they won't deteriorate.