So is this just another marketing sideshow or is it here to stay???
Both.
Came from a combination of mtb companies overshooting the mark when going from 26"->29" and having to come back to a smaller size (27.5/650b). And from road riders wanting to re-enact a randonneuring trend from the 30s/40s as their bodies aged out of high pressure 700c sized wheels. It's not going anywhere because it allows gravel-themed bikes to be designed with the same clearance levels and parts but to fit both 650b/700c wheelsizes in respectable widths.
FWIW, I found 650bx42 to roll slightly slower than 700cx42. Noth enough to bother me in my regular riding but significant anytime I was riding in an event or with other riders. Or when the terrain was especially bumpy. Or at the end of very long or very hard rides.
It's about the same as 26" and 29" in the mtb world, rollover, moment of inertia etc. There are differences but mostly it's preference with a splash of what your physiology prefers and how tall you are.
650b is much closer to 26" than it is to 700c:
