I'm a big fan of the 'look twice, save a life concept' but motorcyclists can be more adequately described as suicidal than the typical driver can be described homicidal.
I've been in a few situations where I had to work very hard to not kill/injure someone because a motor cyclist was being completely reckless. It is super annoying to have motorcyclist squeeze around you on both sides on a one lane ramp merging info traffic. Or a motorcycle sitting in my blind spot without even the decency to stay near the center of the lane.
Its not really different from any other driver, except the complete absurdity of driving among cars at car speed with literally no safety mechanism beside plastic wrapping your head should demand a heightened level of defensive driving.
I've frequently seen motorcyclists acting like they're suicidal (just not quite deliberately riding under the front of that lorry), drivers acting like they think they're invincible, cyclists acting like nobody else on the road is actually there so they don't need to bother trying to avoid being run over by them, and pedestrians who think the traffic will just magically avoid them when they run across the road in the dark while dressed all in black.
There are terrible people in control of all vehicles.
There are definitely the types that seem to think the streets are a racetrack and are very wreckless. I'm one of the 'other' types and I ride like a grandma and try to stick to open roads.
BTW, there are good reasons why motorcyclists don't always drive in the center of the lane. One main one is to position the bike so you are visible in the left mirror of a driver you are passing on the left, meaning you ride in the 'right track' of the lane. Another is oil and gas tend to be spilt in the center of the road, and most bikes don't have ABS. Riding in the left track will help prevent a driver passing you on the left from cutting their pass short into your braking zone. It all depends on the surroundings and riding defensively.
I'm a motorcyclist as well and I too tend to ride quite defensively. Another reason to avoid the centre of the lane is that the arrow markings tend to be slippery at times - especially on wet roads. The biggest factors, though, are the ones you mention - to be visible in the mirrors and to avoid fluids that have leaked from bigger vehicles - which usually is in the centre of the lane.
