Stable addresses across reboot may be necessary. I want to be able to build apps that take advantage of the large IPv6 address space and consider that addresses are stable.
So, you want to build a simple application. Don't we all.. :) But I don't think this is something we can guarantee even with native access. ISPs just aren't offering static v4 addresses today even when they'd have technical means to do so. So, counting on static v6 prefixes would IMHO be cutting corners too much.
The problem is worse with transition mechanisms, especially the ones which traverse NATs, but the situation may improve as soon as we can get rid of them. In any case, such mechanisms can provide a stable as long as they can keep the NAT/IP mappings stable -- which is, for a properly designed application, maybe sufficient.
On the other hand, as long as John Doe's home PC stays powered on and connected to the Internet, his address should stay stable.
I suppose that you are aware of ISPs that change DHCPv4 allocated addresses every 24 hours... If John Doe's computer build its v6 address form those volatile v4 addresses, its v6 address will also change very 24 hours, and this even though John Doe has not rebooted his PC. Not good.
Yep, I know this -- unfortunately. But there is little that can be done to fix ISPs which want to enforce the users not to do Foo (whatever Foo may be). We can hardly fix the misbehaving ISP problem at the IETF.. as you're probably well aware :)