Split pins missing. Mechanic 1 told me you must do it, it is essential for safety. Mechanic 2 told me its no big deal only japanese cars have it, you don't see them on german cars and car is still secure without it so if you don't replace it's no real problem.
Never Never Never Question cotter pins, Cotter pins are essential to road way safety
I used to be a axle mechanic for 14 years, In that 14 years I too made the mistake of Questioning Cotter pins, and tab washers. But after seeing what I did over the last few years I would say if you're going to leave a Cotter pin or tab washer off then just leave the hub and tire off also because the vibration from the road is going to spin that castle nut right off within the next few miles, after the castle nut is off then you my as well say good bye to your tire. Imagine a tire coming at at 70mph, how many lives are at risk including your own...
Cotter pins are a must have, no question.
There was this one guy right before i left that came in with a flat bed trailer, the trailer he was hauling had a car on it and a bunch of random salvage yard junk. But to make a long story short, he lost his Cotter pin on one of the dual axles and the other side was hanging on by a thread. His axle was completely ruined. 70mph down the highway with no tire does good damage... the Cotter pin that was barely hanging on the opposite side was bent in 2 places, one of the bends broke... this didn't tell me that he got a new Cotter pin and bent it down, this was a clear sign that he re-used and old one... once you bend a Cotter pin back straight you ruined it. It's garbage, don't take the risk
The only thing that kept this guy and the others around him alive is that he had a Dual axle, If he would of had a single 2500 - 6500 pound axle then he could of really hurt someone. Cars, trucks, suvs are single axles, but buses, semi's, pontoon trailers are duallies. But that don't mean the risk ain't still there
Yes, even nylon nuts will vibrate off, they'll just take longer than a pressed nut
Mechanic 1 is right. If there is a hole for a split pin then the car designers felt that there was a serious need to keep that nut in place, not a "oh well the AC quit" or "what's that rattle?" kind of need but a "going off the road and dying" kind of need.
The only time I wouldn't see replacing the split pin is when using a self locking nut, in aviation mechanics you can use a locking nut in place of a cotter pin on flight control surfaces and if it's good enough for a certified airplane it's good enough for a car as long as the nut is replaced every time (it would be worth checking the manufacturers spec before doing it though). However, I'd still put a pin on, it takes seconds and is extra security.
If the car has one (e.g on a ball joint bolt), then put one back on. They're very cheap - a box of various sizes will last forever, and costs only a couple of dollars.
Essential? Not if everything is torqued to spec. But it's cheap security, just in case.