If they are used correctly then they could be very useful.
For instance there is no reason as to why a chicken recipe that is also gluten free could not be under both the gluten free area or chicken, Turkey and other fowl. So where do you put it? It's not specifically gluten free, just happens to be so, so normally you would put it under chicken... If it has both the chicken and the gluten free tags and someone is looking for a gluten free recipe then using the gluten free tag as a search will show the recipe if it is under chicken... as well as if it was under gluten free.
Ordinarily I would only expect recipes that have specific substitutions to make then gluten free (so gluten free dumplings) to be under gluten free and not recipes that are accidentally gluten free. (I'll accept that this does not work as well from a vegetable and vegetarian point of view but...)
So I think they can work if people use what is there and don't create duplicates (so plurals where the singular is there (cookie/cookies spring to mind)). There are plenty of recipes that are accidentally gluten free or vegetarian/vegan, wheat free etc that even someone who is not wheat free or vegetarian can clearly identify. Plus anyone can add tags to a thread, not just the person who created the thread, so this will help create a tags database if we all add them as we find something missing.