I frequently export field definitions so I can programmatically create them in an update.php function. In Drupal 6, the CCK module came bundled with a Content Copy, but (so far) I have found no similar tool in Drupal 7.
A friend of mine asked how to create a link programmatically in Drupal. I do this sort of stuff all the time, because one of the projects I help with requires us to write update scripts for our deployments.