Why?
EDIT: I've previously said on HN that I'd like to buy Flickr and roll it into a benefit corp/non-profit sort of deal. I firmly believe that Pro users can pay enough that Flickr can self-sustain itself.
Integrate with Tumblr has a good upshot - browsing Flickr picture is kind of dull. If you have a Medium-like site for photo publish, you can create a web photo magainze!
The one - on Tumblr is all the custom themes. IDKY, but Medium just looks better for readers. Yahoo needs to make Tumblr for high quality posts, but Tumblt started out with teens, mostly asks, gifts and quotes. Now really just about reblogging every day and like I said I keep seeing the same stuff reblogged every day.