quoting Hallow XIII:

the benefits of tagging lie in collecting posts where the word in question is not contained or encapsulated in a string that is not conducive to search, but that is relevant to the topic (example: posts on physics often do not contain the string "physics", and posts on e.g. Kalmyk are about mongolic languages even if they will tend to not include this word)

This is a reasonable argument; however, if we are to have this, my strong suggestion is that we do not make the system independent of searching  - that is, any search should turn up both post contents and any tags on the post. This way, nobody has to add tags for words that are already otherwise in a post; then someone posting about Japanese pronouns, say, need not additionally tag it with #Japanese and #Pronouns because those words are (for the sake of argument) already in the post, they need merely tag anything else it might be relevant to that isn't explicitly mentioned.