[greyface-]

You need to be aware of it to make v4 work, too. I once saw an issue where a network similar to your example WISP (singlehomed behind HE) was unable to reach a network that was advertising its v4 prefixes NO_EXPORT to HE at a distant IX. Adding a second upstream would have fixed (and indeed, did later fix) the issue. It's been advisable since the early days to have multiple upstreams because of routing table holes created by situations like this. Again, not unique to v6, and the HE/Cogent schism is not the only one (though admittedly it is likely the largest).