While Twitters not making that much right now (at least compared to Facebook, etc.), they're clearly moving toward more aggressive monetization (for example, see Twitter's ad products and how advertising is key).
In advertising, audience share equals money. If Twitter gets shut out of Search Plus Your World results, and more traffic is driven to Google+, it will undercut their bottom line.
And in case we forget exactly how much money this can mean, there’s a nifty infographic on Facebook’s IPO. It’s all interesting, but here’s a snippet: