In relation to the relationship between durability and clarity -
One thing that we look at in 'studies' is whether there is a statistically significant difference in a result. For instance, a X% increased risk of breaking a diamond because it has Y clarity. So the questions are:
1. Is an IF stone less prone to breakage compared to one of a lesser grade?
2. Is this observation (if it's true) statistically significant and consistently reproducible or purely due to chance?
3. More importantly, does it matter? For instance some people may say Y is 50% more likely than X when the real figures show the real figures of X happening is 0.01% and Y is 0.015% (hence 50% increase!). So is an IF stone that less likely to break compared to a VVS2? Or how about VS2 / SI1? But would this difference influence our decisions and our setting choices in real life?
In medicine, there are multi-centre randomised controlled trials where large amounts of data are collated, variables and confounding factors identified and then inferences made based on the data results. Pity it wouldn't be practical to design such a study here ... and thus the durability question may never be fully answered. Until there is 'hard evidence', people will always stand by their own personal experiences and opinions. Makes for exciting heated discussions and debates
Although, just as controversial - the GIA attempted to design then implemented a 'large expensive' study in diamond cut/brilliance etc... that they now apply in their certificates.
I know people who have bought an IF stone. The reasons may be simply practical rather than the ones listed (A buyer may want a D-F coloured stone in a particular Ct weight and cut quality. The vendor only has that criteria with IF clarity 'in stock' and a lower clarity and cheaper stone wasn't and if one had the money and couldn't wait ... plus loved the stone on sight - then they simply paid the extra dollars)
On an aside, High clarity, colourless stones with excellent cut and performance are exceptionally hard to find. Do a search on PS for a D-F , <VVS (or even IF) that is ACA H&A and you'd be hard pressed to locate many stones at all. Most vendors stock what their market want to buy. In the US and Australia, I would assume that would be 'value for money stones' G-J ; VS - SI1.
On a side note, I also notice with interest there are not many AGS cert diamonds in Australia.