I've collected a lot of images over the last 8 years of digital photography, using a variety of increasingly acceptable cameras, processing them using software that has improved enormously over the years. At times I have given in to encouragement to delete the original images for taking up space (and I'm sure I lost some in a HDD crash some years back). For whatever reason, many original images are no longer available to me.
This is a big deal because I would really like to go back and re-work some of those pictures, saving them as larger files for printing, straightening horizons and verticals and generally cleaning them up now I have both the tools and experience to do a better job. Sadly, for many, it's too late.
So let me encourage you - NEVER delete the excess images you have on your hard drive - you never know whether you'll need them again.