When parking on a hill with an automatic, I prefer the warm and fuzzy feeling that comes with redundant stopping mechanisms (both). This is particularly true when I have a 3500lb boat attached.
I apply the parking brake first. I do not like the balky "feel" of the shift out of park when the car is hanging on the tranny. I figure if it is that hard to shift, it just cannot be a good thing. Will this actually cause real damage in my lifetime? I do not know...
More important, if you live in an ice belt, remember that the rear diff (depending upon the diff in question) is still open when the tranny is in Park. This means that if either rear wheel does not have traction, the car will slide. The parking brake engages both wheels, so this can make the situation better.
In my state, you cannot pass inspection without a functioning parking brake anyways.
I have indeed seen tranny park mechanisms fail. This is usually caused more directly by someone shifting into Park while the car is still rolling, if they would 'fess' up to it...
Actually, I just drove an old pickup this weekend with a marginal ability to hold in Park. Sometimes it holds, sometimes it does not. Of course, it is 36 years old...
1986 300E 5-Speed 240k mi.