Chesten hit the nail on the head. I can't really comment on WC trapping, but in JKD the idea is to just get the limb out of the way so you can hit the guy. I've used it in full contact sparring with some success, but much like any other technique, it's value is limited in some situations.
Knowing what trapping is supposed to do and practicing accordingly is the biggest factor in whether or not you'll be able to make it work for you in the right situations. If you train trapping for the sake of trapping, you're going to spend a lot of time chasing limbs in vain. If you train trapping as one of the five ways of attack in JKD, you're going to be using it just like your other ways of finding / making an opening
Trapping is not parrying. Parrying redirects the opponent's attack and that's about it. It's passive and relies on your opponent to do something first. You can turn a parry into a trap yes, but trapping is active, it doesn't require your opponent to do anything at all. You can trap their attack, you can trap their retreat, you can trap their hesitation, the initiative is yours.
The clips of trapping that Chesten put up are great examples of defensive trapping, but trapping can be offensive too. If you can get someone to cover up, or even better, extend their arms on your attack, you can then open them up for hitting. You'll see a lot of this happen in ground and pound situations, where the person in the dominant position is trying to get their opponent's arms out of the way so they can punch/elbow/etc.