having been an avid downhiller when I was younger and coming off my bike countless times i can assure you that a helmet is essential wear when on a bike. I have hit my head off trees, rocks, the ground, other cyclists and in one amusing incident I kneed myself in the face. Suffice to say I wasn't wearing a helmet for one of them, and after that I always wore a skid lid.
I feel this issue is six of one and half a dozen of the other, I have seen many idiots on bikes dicking around and generally presenting themselves as a hazard to everyone, as well as mentioned above no lights / black clothes etc.
On the other side I have been on the receiving end of what is quite frankly appauling driving and a poor attitude to cyclists and I find it very hard to understand. Every cyclist (within reason) is another potential driver to clog up the roads and yet there they are, a mere inconvenience for the amount of time it takes to overtake them, often less than 30 seconds.
As ever it boils down to the minority of fools affecting the majority of sensible people who wish to return home in the same state they were in when they set off for their journey, transported by whatever means they choose.
Your comments about cycle paths being irritatingly sporadic is something I really have to agree with you about, it's very frustrating.
Finally, cycling on the road + iPod (or equivelent) = FAIL