This is mostly true, but there's two points I'd like to bring up.
As Brucker pointed out, most people who add tags usually add the entry name to the tags. This is one way that helps to easily identify untagged images, by searching for all the images in the gallery that lack the image gallery's name. Of course this isn't 100% effective, but it's often a good place to start if you want to help tag a gallery.
The other is that redundant tags like this is sorta one way people could find entries on the site, due to related images. Related images below the main image are displayed due to having at least one tag the same. If an image uploaded in a certain gallery has an "xkcd" Images from the xkcd gallery will show up in related images, but it doesn't happen if the tag is not on the xkcd image. I know I've discovered a few entries on the site with this.
Nether of these I think are really important in terms of being able to just search images on the site for the average user, but I wouldn't go so far as to say they are completely useless for those reasons.
Don't get me wrong, some galleries it's completely useless to add the entry name (reaction faces, alternative universe, and tumblr come to mind) and if you are literally only going to add the entry name as a tag, I'd rather you not do it at all so they would be easier to find for people who want to add more useful/legitimate tags. (If this thread is trying dissuade users from only tagging with the entry name, then I agree 100%.)