When is it beneficial to freeze the discard pile?
When your team doesn't have enough points to start, but the other team has started, they will be able to take the pile more often and you don't want to keep cards because they have them open.

Sure, freezing the Box costs a wild card, but a lot of time I prefer to lose a wild card instead of giving the box to my opponents over and over.

Also a really good strategy is to freeze the box to let it grow; you keep a trap in your hand to get the box.