> very few people care about squeezing out every last millimeter of cooler clearance

I believe that may not be entirely true, given the recent interest in DIY cheap steam boxes that are packed to the millimeter with fan cooling. While it’s not necessarily an eATX solution, certainly there is at least medium demand for knowing case tolerances within a millimeter these days. 5mm of extra fan thickness gives you something like +10-20% CFM, which can be a performance-limiting factor in these custom builds:

ASRock BC-250: Building the Budget Steam Machine (8 days ago, 51 comments) https://news.ycombinator.com/item?id=49140213