Maybe I haven't been reading much, but what else has Yahoo done recently besides bring in Mayer and do a website redesign?
Tumblr will pay for itself? Not a chance they're going to earn a billion dollars off of that, much less turn a healthy profit. As such, at it seems like all Mayer is doing is burning value on a spend $3 get $1 back exchange. Works great in a stock market bubble of course.