Back Of The Head

What exactly is it about blows to the back of the head that necessitates banning them in MMA? Are they more likely to result in a KO (or, more importantly, permanent damage) than blows to the temple or chin?

What got me thinking about this was the Cole VS Joe Lauzon fight from the Ultimate Fighter, and the fact that Cole was so visibly hurt by the blow. But then again, that elbow would also have hurt if it struck the temple. . .

Blows that at least glance the back of the head are almost unavoidable when the opponent turtles, and it seems like at least half of successful soccer kicks strike the back of the head. Is it silly to have a rule that is almost always at least slightly tread upon in the fight? Have you ever seen a (long) fight where Big John didn't warn the fighters at least once to watch the back of the head?

And if it's such a sensitive area, you'd think it would be possible to knock someone out from within your guard with an elbow or palm strike, particularly if they are controlling your head, but I've never seen it. And it certainly seemed to make Frank Shamrock's knees from the bottom more effective against Renzo, though again, they might be just as effective from the temple.