Hello,
Don't forget that fibre channel is more than just initiators and targets. There's a network in between that must be able to switch/route, address, scale, and provide management, control, and security. Node names are used as the name implies, to identify devices in the network. HBAs are not the only devices. FC switches have WWNNs, as well. Just like an HBA has ports that are assigned WWPNs, FC switches have ports that are also assigned WWPNs. Just from this basic understanding, you can probably see how zoning based on WWNNs is not what was intended by the creators of the protocol. You'd probably get a kick out of reading some of the original FC standards. It's dry, sure, but if you really want to know how "stuff" ticks, go read them. In short, you don't zone by WWNNs because that's not how it works.
This question reminds me of when I was studying my for CCNA. I had a buddy ask the instructor, "Why can't we just route by MAC address?" Of course, this was before TRILL. The answer was, that's not how it works.
I thought I had a grasp on FC after reading Cisco docs and even implementing it, but then I found Rick Mur's Fibre Channel 101 video. It does a much better job of explaining things than reading. CCIE Data Center :: FiberChannel 101 - YouTube
All the best!
Mike
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Mike Brown
VMware, Cisco Data Center, and NetApp dude
Consulting Engineer
Twitter: @VirtuallyMikeB
Blog: http://VirtuallyMikeBrown.com
LinkedIn: http://LinkedIn.com/in/michaelbbrown