You miss key points
1. You cannot compare a head injury to cuts and scratches on an arm or leg.
2. I'm agreeing it's a good idea if you really are on the edge. However when I see some tubby biffer on his £6K carbon bling machine wheezing his way round follow the dog at cannock then I'd question the need for pads and contend it's more a 'fashion' thing.
In my defence, I did say that the idea of wearing pads was an extension to the idea of wearing a helmet (ie. protection in the event of a fall). I didn't say that it was the same thing so I'm not comparing head injuries and limb injuries as one and the same.
Also, pads have got a LOT easier and more comfortable to wear over the past few years so there's less of a reason not to wear them when out riding.
But yes, as you mention, there will always be those who wear them purely as a fashion statement. They usually buy them at the same time as they buy their 29er