Some of this things I read in books and some of them took me few days of troubleshooting and sweating to get to them so I give them for free here to save you fellow networker some time:
The mighty SLAAC is the prefered method of IPv6 allocation, but is it so mighty? Or it only seems to be mighty and magic? Your computers or mobile phones in order to use SLAAC must be convinced to do so by the router RA message. That message includes the A flag set besides the prefix and all other info. That kind of RA message will tell the device receiving the RA that he needs to make the “A” autoconfiguration on his interface using EUI-64 method.
But that’s not all.
RA messages will need to have also the O flag set. With the O flag end hosts will tell the router that they will use DHCP but only for the “O” other options. In the first place that other option will be DNS server IPv6 address which is not possible to get from router RA messages. Why? I’m sure that’s the most frequent IPv6 question. The fellows who made the RFC 4861 documents didn’t put that option inside RA Router Advertisement Message Format.
I did try to find a reason why not. Maybe the only partially reasonable answer is that DNS is hierarchical system that needs to be centralized inside a network architecture and routers as devices that are running routing processes are distributed system (at least before we see SDN in real life). So the answer will be that is not okay to put allocation of DNS address rule on a system that is not centralized. It means that if you need to change DNS in a network with a lot of routers that are sending RA messages on their local subnets you would need to change the config on all routers one by one. That is the best answer that I did find until now, but this sounds more like excuse that a real reason for this decision. If you put all the info together the answer that fellows from RFC 4861 did actually made the wrong decision is in existence of fairly new RFC 6106 that proposes addition od DNS IPv6 address allocation in RA message.