Do we need this tag (84 instances)?

For the most part it's simply being used to indicate sexual intercourse with vaginal penetration and no other connotation (as in breeding cattle for the purpose of producing more cattle). This pretty much describes probably about half of the images depicting sexual intercourse on this site.

To hell with it!

Is it ( or at least supposed to be applied to) on images where the idea is reproduction like cattle?

Well, seeing that no wiki page exists for "breeding", there's not a defined meaning for this tag. I'd have no problem with the it except that it's being widely used as a synonym for "intercourse" in the general sense of the term, and not for breeding as an intentional reproductive practice.

There are, in fact, only a very few images, compared to 68 that come up in a search, that can even remotely be considered "breeding" in the sense that most people probably use the word.

I think the best thing would be to simply nuke it, or have someone carefully go through all those images and figure out just which ones involve intentional reproduction. Even so, the word is generally used to refer to commercial breeding, and not just a married couple trying to have a child or insemination that's merely incidental to having sex.