Well as a motorcyclist I've often heard how dangerous it is, "organ-donor" is a common refrain. Always from cagers, of course.
The consensus seems to be (in my experience) that motorcyclists are just stupid, as opposed to cyclists who are stupid and have no right to be on the road.
As a practical matter however, there is a certain chunk of people who are just cowardly bullies, and cyclists are especially vulnerable and easy to escape when said bully is operating a car. Motorcyclists are similarly vulnerable physically, but generally have no problem catching said bullies, at which point the motorist faces certain repercussions for their actions. Even if it's just getting yelled at, at least it makes an impression.