...Actually, I have a 2nd part to that question:
The reason I noticed it is because I'm moving my wp-content directory, and was searching the db for that string - since it's hardcoded in all these postmeta entries, all of them would need to be updated to the new path - unless it's safe to just delete them (which was my assumption). However, while doing some testing, I did notice incoming hits from useragent "facebookplatform" to plugins/wordbooker/includes/wordbooker_blank.jpg - under the old wp-content path (i.e. the one that the postmeta still points to).
So I guess my question is: what else might this being used for?
(And my recommendation: yep it would be nice not to create them if not being used - and also, you might want to store them as a path relative to WP_CONTENT const, so that if wp-content is moved, the database doesn't need to be changed :))