Lots of opinions. It seems they fall into the category of it's okay to pass another biker in his lane, or it scarred the holy crap out of me.
I suspect those that feel it is perfectly permissible have never had it happen to them. I had one Harley rider follow me in the HOV lane for a good 15 minutes. He hung back and I put him out of my mind as he didn't seem to be a danger. Just as the HOV lane was to split off onto what they locally call the I-5 Express Lanes, he went by me on my left at about 80 mph...then cut over two lanes with a big wave. I had no idea he was going to do that. If I'd moved over even a foot to avoid something on the road, there's no way he could've avoided hitting me.
If he gave me warning that he was going to pass, it wasn't in a manner I could be expected to know it.
On another time, I was coming home from a long trip. Tired; I'm probably at over 10 hours of riding so far with another hour to go. In very heavy traffic on I-405. The road is packed, but moving along at about 70 mph (in a 60 speed zone). Two riders come working their way through the traffic. After a minute or so, the first rider passes me on the left with no warning. Scared the crap out of me. Second rider comes up a minute or so later and tries to do the same thing. At this point, I'm pissed. I move over to the left. After a few minutes of this, he passes on the shoulder...mind you, we are all going 70 mph in a 60...flips me the finger and pops a wheelie in the middle of the traffic.
Just because you can do something, doesn't mean you should.
I don't mind letting riders by me, and I do it often. But for someone to assume I even know they are there...that's wrong and there's no justification for it. You cannot see into my head to know what's going on there. Have I been riding so long that I'm getting loggy and should've pulled off...but have no good options? Am I looking down at my GPS trying to figure out why it is routing me somewhere? Do I have a malfunction on my bike that's diverting my attention? You have no idea.
If you're passing in my lane and didn't get an okay signal from me, you are treating me as a pylon on your personal race course.
Chris