Sorry, Been in this field 30 years and am amazed how everything has to be so "safe" nowadays. To me the real key to safety is taking responsibility for your own safety and not relying on some-one else to keep you safe. If you can't do that no amount of safeguards will save you, you are doomed to fail. long story short
If you've been in this field 30 years then I'm guessing you've probably seen your fair share of welded contacts. If not, then you've been awfully lucky. How safe is an old-style non-redundant E-Stop circuit when the MCR contacts are welded shut? It is true that it is impossible to make any machine 100% safe, but just because a problem cannot be 100% solved doesn't mean that trying to reduce it as much as you can isn't worthwhile.
It's easy to say people should be responsible for their own safety. But there are mitigating factors. What about accidents? Trips and falls? Someone horseplaying around you and accidentally pushing you? Are you honestly saying you've never, ever, done anything unsafe while on the job? If you have, and hadn't been so lucky, wouldn't you rather have a close call than a lost limb?
Do you think these standards are created by a bunch of people who sit up in an office somewhere and draw their crazy ideas out of a hat? These standards actually arise from accidents that really have happened and could have been prevented. A machine can run for 30 years without hurting anyone, and then on 30 years and 1 day kill somebody. After that, those 30-years of injury-free running mean NOTHING.
The thing about safety sensors is that they are far more reliable than any procedure enacted by people. No safety circuit is infallible, BUT the higher categories are far LESS fallible than relying on human beings to perfectly follow all safety procedures while operating a machine. It's common knowledge that the most dangerous people on a job are those who have been doing it for a long time. It's so easy to lose respect for how dangerous a machine can be, and if you run one machine for 10, 20, 30 years and never get hurt, it is incredibly difficult to maintain the same level of caution you had on day one.
It's hard to see the value of safety design because it is hard to quantify how many accidents DIDN'T happen because of the circuit. But in most cases, all it takes is one or two prevented accidents to make the time and initial expense worth it.
EDIT: And just because there are far more elaborate safety circuit standard these days doesn't mean that one's responsibility for his own safety and the procedures involved are being thrown out the window. Machines still have to be locked and tagged-out. Appropriate PPE still has to be worn. Energy sources still have to be isolated.
