Even "dead" snakes can be dangerous:
http://www.msnbc.msn.com/id/20212558/

I've heard of this happening. There's even an old horror story about a rattle snake fang stuck in a boot. The man dies from the bite, and his widow gives the boots to two more husbands and each dies before she figures it out. I don't know that a single fang embedded in a boot could cause that much damage, but I have heard that the head and fangs still contain poison after death.

The fang in boot story is an urban myth. Fangs are mere needles with venom sacks in the head acting as syringe. These stories always fail to mention the retiring nature of most snakes and their vital role in controlling rodents. I've been dry bit twice and do not begrudge the animal for being what it is. Ironicaly, several states have 'rattlesnake roundups' with the happy result of rattlesnakes who don't rattle passing this genetic trait into the surviving populations. Those vastly depleted populations are therefore potentialy more dangerous, along with the explosion of rodents carrying disease and damaging crops.

The old wife's tale was to bury the snake's head till it's dead.
IIRc this reflex bite can occur for up to 24 hours after a snake's head has been removed.

Legends usually have some basis in truth. While the urban legend/horror story may not have happened the lesson that dead snakes aren't necessarily harmless is a good one. I personally don't have any problem with snakes and think that people who kill them indiscriminately are acting out of childish fear born of ignorance. Like you point out, better a handful of snakes then mice and rats.