I think that the "add people" function is useful. Imagine to add 100 new photos to your library: I think that is handy to have a page with a list of people belonging to those image which the AI was not able to recognize. Than a human action is required: you just need to choose if you want to group them, give them a name, assign them to a contact or to hide them.
I agree with you that you should be able to hide the contact from that list, instead of adding to the list of people and then hide them.
These actions should be streamlined. It seems that they had not enough time...
Pay attention that if you manually remove unwanted faces on the photos (using the same method that was available under El Capitan), you end in having blank thumbnails under the people album. I have opened a discussion here: Blank thumbnails for people under macOS Sierra