The RFC provides for two address ranges in fc00::/7, one for random prefixes (fc00::/8), the other reserved for later management (fd00::/8).
Later, in some undefined way. A PI lacking enterprise considering doing v6 this way either waits or decides the available space will do as someone will fix the managment later. Sixxs demonstrated that some will see a need With low take up of v6 it's early to know what they will see important
The more important it is to you that your allocation be unique, the more careful you will be to choose a truly random one.
So a way to have really unique is reasonable.
The chance that any random prefix will conflict with any chosen prefix is very, very small. The chance that two conflicting prefixes would belong to entities that will ever actually interact is even smaller.
People still play the lotteries. brandon