Technique should be about two things, health and efficiency, health being priority number one. It can be perfectly healthy to have the thumb poke out over the top of the neck, not very efficient likely, but plenty healthy. A significant number of individuals (including a certain Mr. Wooten) do this all the time to no detrimental effect.
The main reasons for keeping the thumb behind the neck are reach and freedom of motion, and it has less to do with preventing injury. Sure, we've all let our thumbs go over the neck occasionally with no detriment. But there's no positive benefit from doing it. May not injure you, but it certainly doesn't help you play better.
Going to look over Fergie's link here before I comment further...
EDIT: OK, just read his link. Aside from this silliness about holding the palm flat out in front of you, his RH thumb is definitely NOT out to the side in any of his playing pics. I would call his RH technique pretty good if the pics are any indications...he doesn't put his hand in any stressful positions. So at the very least, his flat palm/thumb to the side thing is completely irrelevant to playing bass, and doesn't describe at all what he's doing in the pics. But again, when you look at it, his technique is pretty good. If I were Fergie, I'd totally throw out all that nonsense and put in a more accurate description of what he's doing. The misunderstanding he created by the description could very well lead to serious injury.
The LH, however, is horrendous and could actually lead to problems because he's got the bottom of the neck resting on his palm, and the thumb hanging over the top of the neck. At the very least, this decreases mobility and accuracy by a longshot. The palm will drag on the neck and slow you down. The fingers are also no longer curved, and they're all bunched up together, as evidenced by the pics under item #6 on his blog. Plus he's got a death grip going, which absolutely WILL result in injuries. The proper way to do it to curve your fingers so that the meat of the pad of the fingertips hit the strings, keep the palm completely off the neck, and use the thumb to balance your hand and apply just enough pressure to be able to fret the neck, and no more.
So I stand totally 100% behind my original assessment that Fergie is spreading at the very least inaccurate info about good technique, and potentially dangerous information. Fergie, again, I know your intentions are good, but putting this forth as good technique is wrong. There's not even anything to debate. Your explanations are wrong, your conclusions are based on irrelevant info, your LH technique is awful, and you will not get any academically skilled bassist to say that it's right. I would suggest you stop putting this stuff out there before someone actually gets the idea that you know what you're doing. And I'm not saying that to be mean...I'm saying it to stop people from getting injured.