At this time, having spoken to Jonathan Lemon of the FreeBSD project, I know that FreeBSD's support for checksum offloading will permit the capture and injection of packets with a bad checksum. This is something libpcap doesn't have. Support for these protocols within Libnet is something we'd certainly like to see in future. There has been some minimal discussion of this on the mailing list, and this is one reason why this consultation document is being rolled out. Adding these would generally involve making changes to the API, particularly for IPv6.
Portability encourages ubiquity.
Libnet is already an excellent library. Let's remain dedicated to doing everything we can to make it rule the world.