Functional bikes. Not Porn not Anti

That's pithy but not very scientific.

Pure rolling resistance has been investigated because it is easily isolated and measured. Until there has been research into the variation in effort required to move tyres of varying sizes and pressures (isolating variables) over obstacles, we cannot know that fat tyres are consistently better at this or whether any performance gains are a result of size or pressure differences. Moreover, once this is established, it then remains to be seen whether rolling resistance losses are sufficiently offset by vertical motion gains to make fat tyres worthwhile overall. It becomes a case of finding the optimal trade off.

I'll be riding Eroica Britannia in a few weeks on 35mm Compass tyres, so I'll let you know how it pans out.