Joe rightly says it is meant to compliment the unicast stub routing issue and looking at the 3850 configuration guide it says you must use unicast stub routing to use it.
This is slightly different than the 3650/3750 guide where it says you should use it but you could actually configure static multicast routes to bypass it and so there may have been a workaround.
Not being able to test it I can't say.
The PIM stub feature basically says you can only use PIM in passive mode on your SVIs because there is an assumption that there will be no PIM neighbors in those vlans.
So your 3850s should not be transit devices for multicast traffic ie. multicast traffic should not be routed to a 3850 and then on to another L3 device.
But you can configure full PIM on L3 routed ports.
The idea is as Joe again points out that you connect your 3850s to a L3 device running full PIM and then multicast traffic can be routed to and from the 3850s.
Because your 3850s are directly connected using L3 routed ports by the sounds of it I did think you may be able to run full PIM on those and then you could route between the two and with the 3650/3750s this may have worked because as I said above the docs only said you should run unicast stub routing not that you had to.
However the 3850 docs do seem more definitive in saying you have to run it.
Jon
