I would like to propose that, for new versions of WordPress, the default behaviour of the "insert image" popup is to associate no link to the inserted image.
I am working as wordpress consultant and spend most of the time teaching to customers that they have to clean out the LinkURL field. They need to do so on 99% of the cases, and often they forget to, with pages that behave badly.
They simply cannot figure out why an image should be clickable, and naturally they forget about the URL being filled by default. The typical question is: "why this system assumes the image is to be clicked?". The question is correct, the command is named "insert image" not "insert button"
I feel that this "link functionality" is useful just when you are inserting, say, a map and you wold like that when clicking the map's thumbnail the big version of the map is diplayed.
I think that this "map case" is really a typical case where the current default behaviour comes into use, but in the 99% of the cases people are inserting images just for decorative purposes ("insert image" is one thing, "insert clickable image" is another thing).
I really felt like to give the developers this feedback as I think it would greatly improve the system to avoid common mistakes.
All my appreciation for the good work, I recommend WP to 100% of the customers.