Scooper, nicely written and everthing you say about the carbon failure mode is true, but it's not the failure mode that causes the injuries to the cyclist, it's the impact itself. In most cases, you'll be kissing asphalt or metal regardless of whether you're riding carbon fibre or steel. Whether the frame failurs catostophically is immaterial at that point, except to your wallet.

While thinner metals can challenge carbon fibre, you're still likely to come away from any significant impact with a useless frame, despite metals' elongation properties, due to buckling caused by the increase in the diameter to thickness ratio.

However, I will admit that carbon fiber is more prone to latent catostrophic failure after an impact. Frames that look they've survived at crash could failure unexpectedly. This is a legitimate concern, just like it is with helmets.