I've watched it last week - it is very good, illuminating background to how the scene evolved in Detroit and some good clips and interviews. I was amazed when I did the search to find it already on Prime in the UK. Having watched, I now need to delve back into Mark's fine book.

Not any worse than Dan Grissom with Jimmie Lunceford.  And I don't think it is so much a matter of being a "purist". Even tearjerkers can be swung (in a way). But I guess they were part of the repertoire (dictated by popular tastes back then) of the big bands (Black ones included) that they did play at live dates but that otherwise never got recorded (waltzes, etc.).

I will watch it soon!

I agree, this has very often been the case, and the rule rather than the exception. There are countless reissues that reproduced the facsimiles of the front and back covers of the originals but often have a few lines with personnel and recording dates added somewhere (wherever there remained some free space) in the original layout of the back cover. And I do not even count the number of original (or very early rpressing) LPs where I've added the recording dates in pencil (so erasable ) on the back cover somewhere near the lineups.  As for "new jazz", I cannot really comment because there is not much brand new jazz releases I am buying. But overall, among those I did buy (most often CDs) I'd say it's about 50% with and without recording dates. But indeed the recording studio always seems to be mentioned (and the recording dates - if indicated - buried somewhere nearby), as well as the lists of thanks to a milion people out there. 

December 10 Franco Ambrosetti - 1941