Bad driving is the cause of most accidents though, however it's not the people that are weaving in out and out traffic that are the biggest problem IMHO, it's (as Beavis says) the numpties who don't use their mirrors or indicate that are the real issue. If people were in the correct lanes then you simply wouldn't be able to undertake or weave, but that's far too difficult to police apparently.
And no, I don't weave myself no condone it, but done safely with full use your mirrors and awareness of the vehicles around you it's not inherently dangerous.