5.63 per cent jump in click-throughs
Google seem to be suggesting that an increase in "click-through"s implies that users are so happy with the service that they are using it more. I think this is not the case. In fact, I suggest it is the opposite: Since "Instant" is not offering the user exactly what they are looking for, they have to click through more results in order to actually find arrive at their target.
By bombarding the user with constant slightly-relevant-but-not-quite results, the user has to wade through all the noise in order to find what they want, and this may include clicking on a link or two more on the way.
I do find Rosenberg's last comment telling:
"So let me be clear that Instant wasn't based on a narrow financial calculation. We launched it because we could and because it's great for our users, as we have always said"
Note how "because we could" comes first. I fully accept this as the real explanation, just like the development and release of GoogleWave: it's first about a nifty technical challenge and second about upper management attempting to make money out of it. The two not always coincide in user experience, but that last one seems to be tertiary.