Costolo and Mayer obviously know one another from their days together at Google (GOOG). And Twitter is probably a better long-term partner for Yahoo! anyway.
Ultimately, they can help one another out during times of need. Mayer has kicked things off nicely, but she needs a strong platform -- outside of Yahoo! -- to push Flickr forward. And Flickr can pick up exactly where Instagram left off . . . and more.
So maybe I do endorse the idea after all.