Personally I'm inclined to agree. Since Tumblr doesn't really have interest comms the way Livejournal and other blogging sites do, the tags are the primary function by which one can find things that interest them. If you tag a post with a certain tag, it means you want this post to be seen by people looking for that tag -- if you want to tag it for your own personal use, you can just use one of the thirty billion customizeable tags that other people won't be tracking.

Of course, other people don't agree, and I'm hardly the Tumblr tag police, so mostly I just don't track tags.