Ah right, it is used for assigning a single IPv6 address to the WAN interface. I guess this means its lifetimes do not need to be passed to the script since radvd won't have a use for it. 

Now that I am writing it, I suppose it wouldn't make sense for the lifetimes of the IA_NA message to be lower than the lifetimes of the IA_PD message, as you don't want to lose routing over the WAN interface.