We're a loosely connected series of insane reactions that just about doesn't die. We put load on non-load bearing structures (hi, spine), we have our optic nerve running through the centre of our retina giving us a blind spot, our body can decide that peanuts = a threat so bad we should stop breathing and occasionally some of our cells will rebel and try and kill us. We need ultraviolet light to not get rickets but it can also give us cancer. Our fine-tuned digestive tract contains an organ which occasionally bursts. My wisdom teeth have decided that they'd rather grow forwards than up. We don't even produce vitamin C thanks to a single error, something we need to live and could easily produce.

I'm not worried about adding something to my body because it's a perfect machine.

I'm worried about adding it for the same reason I worry changing spaghetti code.

While I agree with your overall point, calling the human body "perfect" is ridiculous. Human bodies, if they were engineered, would never even be released to the beta testers out of fear of the engineers for being labeled as a bunch of quacks. There is much, much to improved on the human body, and we will over the next decades; but I agree that ordering a bunch of syringes from halfway across the world through an anonymous email account and injecting that into your muscles is probably not the way to go.

so we agree. Also let's not forget that the body is engineered to survive in much different conditions than today, never know what will happen tomorrow. So if 50 years from now, after all these tech advances, one says "we don't need the fingers, let's 'cut them out'" I wouldn't rush to do it.

"no way in hell am I going to add all of the sudden"

This usually rapidly morphs into a discussion about paleo diet and such, which I follow but some irrationally hate (or are hired to hate, or whatever).

Direct intervention by human minds/means don't always follow that optimal path, and so for what you gain, there are probably more trade-offs.