Perhaps I am behind the times or perhaps these guys are very forward looking, but I had imagined if you were talking about high densities of servers (vs high densities of cores in monolithic servers) per U I would have imagined you wouldn't have needed high capacity links. I would have imagined you would have just had dual-GigE per server and then you would need a backbone which was un-contended.
I don't know, perhaps a half-U machine needs 10Gb, but that is an awfully wide pipe, more used for bulk transfers than for fast transactions? As I say, I don't know, probably these days 2GB/sec isn't fast enough for a compact many-core transactional server.
Feel free to correct my ignorance.