If Yahoo wanted to more tightly integrate with Tumblr, then why a complete buyout? It makes far more sense to partner with Tumblr at a lower cost. If Yahoo plans to bet the farm on Tumblr add to the legal agreement that in the event Tumblr goes under, that Yahoo retains the rights to the technology. Unless Tumblr already has that in place with another entity, which means an out and out buyout is required. They could have had a partnership where by if Tumblr falls below a market value, then Yahoo has first buy out option. Which circles back that Yahoo doesn't feel that it can partner with Tumblr in fear of lost assets and sunk costs, because there is a real potential of the Tumblr not succeeding in the future. This is not a case to flop one billion dollars on the table.
I understand the model Yahoo is fielding with Tumblr, many companies do this. But they mostly do this in areas where the "mother ship" has no direct experience in that market. It's best to keep the business models separate. But for Yahoo who is in the same market space as Tumblr to have a hands off approach is a testament that Yahoo's executives have no idea how to incorporate technology. Which gets back to this is an overpaid partnership.