I think the usefulness of tags varies across different SE sites. Besides StackOverflow and Academia, I am active in Sports.SE, and occasionally drop by English.SE, Fitness.SE and Arqade.
Based on my experience, tags are very handy in Sports.SE where you can easily filter the questions based on your interest and knowledge of the discipline in question. I have tags that I have identified as favorites, and tags I ignore. Makes things much easier...
Arqade is just as self-explanatory as SO, since pretty much every game has its own tag. One simply needs to search for questions tagged with a game of interest, and BAM! you got everything you asked for...
With Academia.SE I think most of the tags are pretty useless, IMHO, since an overwhelming number of questions can be assigned to one of the following:
There is a whole lot of (at least partially-) redundant tags (e.g. publishing/publications/writing)
Pretty much everytime I ask a question I am bothered to find the "right" tag(s), so I feel a clean-up of the existing tags would be beneficial for the site (at least from my perspective)