Chris,
thank you for taking the time to read and respond.
Again, you are bringing zoning back into the scene, so now I'm strongly considering that the reason is indirect, i.e.. it's a security mandated thing.
Yup, I can see the brittleness of zoning using WWNNs. In fact, there seems to be a design "party" that even considers soft zoning as brittle.
My concern was about the loss of (high?) availability for the boot process on a port down situation.
Booting from a WWNN was, in that respect, a superior option.
Obviously you can compensate that by setting two boot targets 
I guess it is clear now.