I keep them for much the same reasons as has been mentioned here - it is nice to see how you have developed.  And to answer one of the original questions - yes - i would re-shoot a shot i thought i could improve upon (and often on my walk arounds in st andrews i do shoot the same scene just incase it looks better than last time i went there ;)  But i wouldnt delete the old one - i would probably link to it in my description to give others a chance to see what i did before and what i did now - that is where it gets interesting to see development and choice :)

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography