Where do you think the mantra of 'smaller class sizes = better education' came from in the first place? Though I don't doubt the advice was was honestly intended, teachers do have an economic interest in reducing class sizes (as it increases demand for their educational skills and accordingly, wages). It's got to be easier to teach 10 kids than 20, and we generally feel we're more productive when we're not overly stressed, the economic implications of the 'small is beautiful' policy were incidental or even unconscious [1].
So, the Gates foundation spent a lot of money discovering that this has little real effect on educational outcomes. I don't regard that as a waste - it may have been an expensive lesson, but I presume the money was spent on subsidizing smaller class sizes in schools. The understanding we've gained will save many billions of taxpayer dollars from being spent on a widespread but mistaken belief about teacher:student ratios, which is a significant public good.
[1] I say 'were', because now that we have evidence of the policy's ineffectiveness, many teachers' union members have designated the Gates Foundation and Obama's education secretary as evil corporatists out to wreck public education.