I don't have the kind of "smoking gun" evidence that Drall has, but I've seen things that make me suspect some kind of throttling. The clearest evidence I've seen is that deliberately reducing my ranking for some terms seems to increase the rankings and traffic I get for other terms.
As for why Google would do it - to create artificial stability. Too much traffic fluctuation would create economic chaos as business revenue would similarly fluctuate, leading to (even more) backlash against Google from the business community. It would make sense for Google to implement measures to increase stability of traffic, especially as they get more into "real time" search which would normally cause increased instability.