If operational simplicity of fixed length node addressing is a
technical reason, then I think it is a compelling one. If you've ever
done any reasonable amount of work with Novell's IPX (or other fixed
length node addressing layer 3 protocols (mainly all of them except
IPv4!)) you'll know what I mean.
I think Ethernet is also another example of the benefits of
spending/"wasting" address space on operational convenience - who needs
46/47 bits for unicast addressing on a single layer 2 network!? If I
recall correctly from bits and pieces I've read about early Ethernet,
the very first versions of Ethernet only had 16 bit node addressing.
They then decided to spend/"waste" bits on addressing to get
operational convenience - "plug and play" layer 2 networking.
The difference is that it doesn't "cost" anything. There are no RIR fees,
there is no justification. You don't pay for, or have to justify, your
Ethernet MAC addresses.
With IPv6, there are certain pressures being placed on ISP's not to be
completely wasteful.
This will compel ISP's to at least consider the issues, and it will most
likely force users to buy into technologies that allow them to do what they
want. And inside a /64, you have sufficient space that there's probably
nothing you can't do. 
... JG
> If operational simplicity of fixed length node addressing is a
> technical reason, then I think it is a compelling one. If you've ever
> done any reasonable amount of work with Novell's IPX (or other fixed
> length node addressing layer 3 protocols (mainly all of them except
> IPv4!)) you'll know what I mean.
>
> I think Ethernet is also another example of the benefits of
> spending/"wasting" address space on operational convenience - who needs
> 46/47 bits for unicast addressing on a single layer 2 network!? If I
> recall correctly from bits and pieces I've read about early Ethernet,
> the very first versions of Ethernet only had 16 bit node addressing.
> They then decided to spend/"waste" bits on addressing to get
> operational convenience - "plug and play" layer 2 networking.
The difference is that it doesn't "cost" anything. There are no RIR fees,
there is no justification. You don't pay for, or have to justify, your
Ethernet MAC addresses.
With IPv6, there are certain pressures being placed on ISP's not to be
completely wasteful.
I don't think there is that difference at all. MAC address allocations
are paid for by the Ethernet chipset/card vendor, and I'm pretty sure
they have to justify their usage before they're allowed to buy another
block. I understand they're US$1250 an OUI, so something must have
happened to prevent somebody buying them all up to hoard them, creating
artificial scarcity, and then charging a market sensitive price for
them, rather than the flat rate they cost now. That's not really any
different to an ISP paying RIR fees, and then indirectly passing those
costs onto their customers.