My turn signal is a warning, not a request.
Merging has become a blood sport. People lack common courtesy and common sense. I have found that size does matter and people that lack common courtesy and/or common sense find it when they're presented with the prospect of being crushed by a vehicle many, many times their weight and size.
I use my mirrors and turn my head, but the reality is that I can't see everything - particularly people trying to squeeze past me at the last possible second to avoid being "stuck" behind me. I will not put my rig into a guard rail or slam on the brakes to avoid the idiots. Coming to a complete stop on an acceleration ramp (that's what they're called in NJ) and then trying to merge is far more dangerous than making some boob apply their brakes or change lanes.