They can use the existing structure of peering and transit agreements to control to whom they are willing to provide service, and possibly to charge for the service. In practice, however, such routes are almost guaranteed not to work. The actual limit may vary from network to network, and also over time. It could indeed be argued that using a large network is a waste of the precious addressing resource. However, this is a waste for the good cause of actually moving to IPv6, i.e. providing a real relief to the address exhaustion problem.
Erroneous advertisements are a frequent source of errors in inter- domain routing.
A misconfigured AS will advertise that it can reach some random network, divert the traffic, and effectively cut that network from some parts of the Internet. As a protection, many managers of border routers use databases to check the relation between the advertised network and the last hop in the AS path. It can be argued that using ND will be more a more standard way to do routing, that with a regular association failures will be easier to detect, and also that nailing the default connection to a specific 6to4 relay router will avoid any transient failure caused by the routing protocol. On the other hand, there are many arguments against this type of association. Using ND to discover the next hop router may be in line with the generic IPv6 architecture, but is actually not in line with the specific software written for 6to4, in which the IPv4 next hop is derived algorithmically from the IPv6 destination address.