So If I answer my phone and loose concentration while driving is it my cars fault for allowing me to drive into oncoming traffic?
What you have just described is a common scenario with many tools, and is not a result of a defect in the tool but in the operator. I have witnessed more than one industrial accident that has transpired exactly as you have said. Most accidents are not with inexperienced users, as they maintain a high level of respect for what they are doing. They are far more likely to happen to the experienced user with many years of experience on the given tool or process. I saw one guy with 30 years of experience in high end furniture and cabinet making loose two fingers on a $35000 sliding table saw. It wasn't the sliding table saws fault that he ran his hand through it, nor was it the company's safety policy's or OSHA's... The user disregarded standard safety procedures and was too pre occupied with finishing the task and made a costly mistake. There was nothing "debatable" is this negligence is this scenario and neither was is the scenario you posted. If your not going to follow the rules that keep you safe, eventually you will pay the price.
I don't think I said it was anything but operator error.
The OP asked for reasons why the 650 is better than the 550. I just wanted to point out that it is much easier to make a mistake with the 550 than it is with the 650. I know you have one, I'm not blaming the machine.