in this question Find IPv6 Router advertising a prefix Mark makes a comment that ipv6 router advertisements are bad. Why is ipv6 traffic on the network bad? (even if it is a rogue router advertisement which it sounds to me like it would only be a problem is you were already routing ipv6) Is this only bad when you are routing ipv6 already?
From the context, I think he meant that a rogue IPv6 router is bad, not IPv6 traffic per se.
One problem with a rogue IPv6 router is that in general, most nodes are configured to prefer IPv6 routing over IPv4 routing when IPv6 is available, so any traffic that could go over IPv6 would go over IPv6, through the rogue router.