There are currently 2 definitions of portrait: 1. close-up of head, neck and shoulders 2. ones like portrait umineko_no_naku_koro_ni I use bust instead if at least, adding to (1.), the breasts are also in the image.
Personally, I use face only for images such as post #880189 or post #803220. Most images which are not portrait or bust shouldn't be tagged face either. With how it is currently done, you could add face to any image with a face somewhere in the image and it couldn't become any more useless than it already is. You can't tell me that post #879983 or post #837579 places "emphasis on the face". Edit: By "we", you mean "you". Feet and face are different, the latter is in 99% of images with chartags:>0
While I agree that a body part need not be focused to be tagged, something like face seems a bit too general to not be constrained in some way. Otherwise, about 99.5% of Danbooru's posts would be applicable to the tag, and that makes it useless to just about everyone.
In the case that face be constrained to images that feature faces prominently or saliently in some way, an implication from portrait won't work.
I kind of agree with Hazuki's tagging of face (since it's nearly entirely his doing). I do think some posts with the tag lack focus (post #884906, post #884649), but from the first few pages at least they're a minority I can comply with.