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What a mix of emotions over the Yahoo/Tumblr deal today. In corners of the New York scene there is celebration at a consumer Web company crossing the symbolically important $1 billion mark (although just barely) while also echoes of sad trombone that one of its two most promising consumer startups (the other being Foursquare) failed to become a standalone company. There’s the typical user fear that something great will be destroyed — prompting Yahoo chief Marissa Mayer’s almost defensive “Don’t worry we won’t screw it up” salvo in confirming the deal.

A lot of the press is questioning that — citing past Yahoo acquisitions like Delicious and Flickr that ruined everything great about what they bought. As if Yahoo itself doesn’t have a blemished enough track record, there’s nearly every other tech giant in the world to point to. But such comparisons offer almost no insight to how this will go for a few reasons.

Don’t consider Yahoo’s track record — consider Mayer’s. What Yahoo needs Tumblr to be is a YouTube. Tumblr gives Yahoo a strong stake in user generated content, a far younger, hipper audience to sell ads to, and ton of potential ad inventory. Then there are all the intangibles. If Yahoo has Tumblr it becomes a more attractive company to go work for or sell to.

Mayer knows better than most what made that YouTube deal work so well, as one of the senior members of the Google brain trust until recent years. She doesn’t have the dysfunction of past Yahoo CEOs, and she’s in a far more desperate situation. At Google, she’s had experience making these things work.

Consider the context of the deal. Those previous deals were done at a time when Yahoo was far less beleaguered than it is today. Yahoo is desperate for a success story. The Yahoo of the mid-2000s was playing around with user-generated content and could afford to squash what it purchased. The Yahoo of today cannot. This has to be the great hope — the future. The glimmer of hip and modern in a company that’s still essentially peddling the same portal product only now on iPhones. Everyone is waiting for Yahoo to fail, and the Yahoo team knows it. If Mayer has any ability at all as a CEO, she won’t let Yahoo ruin Tumblr because of all that’s riding on it. So much of Yahoo’s position in the market isn’t her doing and, sadly, isn’t in her control to fix. This one is.

And then there’s the price tag of the deal. A sub-$50-million deal just doesn’t have the level of strategic importance of a $1 billion deal. It’s a smaller injection of startup DNA into a company. Its business is far less baked if it’s selling for that small of an amount, and its founders likely have less clout. That all makes it harder to fight the inevitable laws of big company physics that squash small teams. There’s symbolic importance of a price tag, and a company that is valued at less than $50 million can die without someone losing their job or a stock price collapsing. If a big company can screw something up without repercussions, it most likely will.

The track record between acquisitions in tech that worked and didn’t work doesn’t always correspond to the amount paid, but they frequently do. Consider the big companies we consider “good” at this — or at least better than Yahoo. Look at eBay: Smaller deals like StumbleUpon were mostly neglected; PayPal was one of the best acquisitions in the history of the Web. And while the monster purchase of Skype made little sense and was spun back out, eBay didn’t destroy it in the process. Look at Google: It’s stunned many people (me included) by keeping YouTube independent and helping it take on even more cultural importance. Meanwhile, smaller bets like Blogger and Dodgeball fared poorly.

When it comes to Amazon, people have criticized the company for doing little with IMDb, but it’s left Zappos alone to keep its own quirky culture customers loved, even indulging Tony Hsieh’s $350 million Downtown Vegas experiment. Even Facebook — one of the best companies at sucking acquired talent into the Facebook borg while killing the acquired products — took a different tack with Instagram. It erred when it introduced ads too aggressively — and quickly back-pedaled.

Don’t get me wrong, I don’t think Tumblr can save Yahoo. I still think the company continues its slide into irrelevance — although maybe a bit slower now. But Yahoo won’t kill Tumblr. It won’t innovate on it much, but it’ll find a way to turn it into a larger business, and it’ll be a good custodian of what users love until something better comes along.