BPF because it's already in the kernel and has a rule language.  Rules to match "all DHCP packets, and IPv6 router solicitations" are easy to write, and people who want to do something different/wierd don't need to rebuild their kernels.

If it makes any more sense, substitute *.*.*.1 for "something matching the IPs I use for gateways".  Aim: make it harder for people to configure their PCs to respond to those ARPs and break the network.  Although for a mobile node whose DHCP lease hasn't yet expired I can see that causing problems.  But if it's just a BPF filter then you can experiment with rules like these fairly easily.