I try to cull mercilessly for the following reasons (in "my" order of importance):
1. I strongly believe that the difference between a good photographer and a bad photographer is that "a good photographer only shows good images". I think this statement applied beyond what you show to the world outside. It applies to what you "show yourself". I feel that I benefit from only keeping around images that are in my own mind "good'. It unclutters my minds vision and keep me more disciplined both in my shooting and editing.
2. I believe that keeping a smaller amount of images makes it easier to manage and keep track of what you have, both in your management system as well as in you "mind". When you need it you pretty much know if you got it or not. Not only that you do in fact have an images of a particular subject, but also that you have a "great" image of it and something you can with confidence, off the cuff' say to a client "yes" rather than "let me go dig in my file-pile".
3. I find that there is a cost of ownership for every file I keep. Yes, you could say that a 1 TB hard drive at $100 is "cheap" , although that is of course a subjective statement as we always seem to think that something that originally cost $X and a year later costs $x -60% is "cheap". But then if you consider that you want to keep for example a) a working set of images b) a backup c) an archive backup and d) an online storage of originals, it suddenly isn't so "cheap" anymore. Especially online backup of RAW files is at least in my mind not "cheap". Yes, you could be diligent and transport your files on HD's off to some off-site location or FTP them to another location you maintain yourself, but pretty much all the options involves cost one way or the other.
Having said this, I still find it difficult to kill some "darlings" and I still need to become more disciplined. But I can say that I have never in hindsight "missed" an image that I doomed to go live in pixel afterlife.
