BTW, the argumentation in 3.5.5 of RFC 6126 seems a bit strange to me. It essentially says that unreachable routes are added to avoid transient routing loops before Babel converges. But transient routing loops until convergence is a common behavior for IGPs (both RIP, OSPF and IS-IS do that), while blackholing may be far less expected behavior, esp. if it is for several minutes, which is far longer time than usually necessary for protocol convergence.
Yes. Babel is designed to be robust not only on wired networks (where OSPF and IS-IS work just fine), but also on wireless mesh networks, where a routing loop, even a transient one, causes cross-link interference and may prevent the routing protocol from reconverging. For that reason, Babel prefers to blackhole rather than risking to create a routing loop.
It seems more like local policy setting than something which should be part of protocol specification.
I agree, it could perhaps be disabled on some kinds of links. -- Juliusz