On the other hand, Tumblr has been bad about monetizing its product in a way that benefits both its users and itself. (David Karp is on the record as saying that he doesn't want to impose a revenue model on users—something he says would hurt creativity—most recently at GigaOm's paidContent Live conference.) I could see Yahoo taking over Tumblr and being more flexible on this front, which could be a good thing for Tumblr creators, many of which have been forced to troll for traffic on other networks despite the fact that their base is on the Tumblr dashboard.
Having attended that conference, it didn't seem clear to me that Karp had a lot of answers as to how to improve the monetization situation for both the company and end users (they've been pushing a blanket ad model that isn't targeted but is focused on exposure-style marketing), so it could be good on that front.
But on the other hand, this community already has a lot of frustrations with the way Tumblr is operated, and a purchase like this could scare much of that audience away.
I'm sure an exit at this point makes sense for the company. But the whole thing sort of bugs me.