When I was helping to develop Corona renderer, we settled at 10 to be reasonable default when we first introduced indirect ray clamping (Max ray intensity), and our userbase at the time was mostly high photorealism ArchViz.
When V-Ray implemented its version of the feature shortly after, the default value was also 10 if I remember correctly.
Now Cycles in Blender also defaults to 10 since it got the feature implemented.
So I find it kind of funny that Lumen, which does quite a lot of light transport accuracy sacrifices to achieve high framerate and stability at the expense of it defaults to as high value as 40, when people doing high end VFX and archviz for the past decade were fine with 10 being the default in their offline path tracers 
IMO it’d be worth it to reduce it to 10 by default for lumen too