Coming here from hippietrail's new question on the same - I agree that this is obviously against StackExchange's own rules and as such the tag rationale should be deleted.
Going by the rationale of "rationale", every single "why" question could be a rationale question. This falls very solidly as an unimportant, unuseful tag.
However, this causes another problem: as far as I know, there's so simple way of "deleting" a tag or nuke it altogether en masse (possibly why it fell through the cracks first time): the only way is any unused tag is automatically deleted at 03:00 UTC every dayany unused tag is automatically deleted at 03:00 UTC every day.
Given the breadth of random questions it has been used for, it cannot easily be merged / synonimised with anything else.
For what it's worth, I think it just needs a community cleanup of manually deleting the tag from every question and then it should die.