If an edit that only adds or removes a tag should be closed as "Too Minor," why do we allow them?
Tags are a very important part of a question. They are what classify it. They are also a big part of finding a question and getting it answered.
They provide visibility in the following ways:
The title and tags of a question are what make the question identifiable, which is why you are allowed to edit both without the character limit.
Approving tag only edits help get a question answered because it improves its visibility in the right places. I'm not saying all tag only edits should be approved, sometimes when there is a lot of other things that need fixing I will reject them, but on the whole they are quite useful.
There was even a privilege that allows you to edit a question's tags without going through the review system (it was since removed, because of "confusion").
If the post is correct and the only problem with it is tags. I'm totally fine with someone doing a tag only edit.
If the post has a lot of other issues well then it's up to reviewers to reject it.
So I don't think we should prevent tag only edits.