I wear a helmet every time I ride, and wear my crash shorts only when I know I'm going to be hitting the park. I thought about wrist guards, but I don't think they're needed. Just learn to fall and you'll be a-ok.
Besides a helmet, I think everyone should wear wrist guards. Regardless of how experienced you are, wrist injuries are common and you are probably more likely to have a bad fall on your wrist when missing a landing off a jump in the park as an experienced rider than a beginner catching an edge on a groomer.
Wrist strains are just a pain in the ass for a few days and that may not be enough reason to wear wrist guards. However, the wrist is a beautiful union of 8 small bones that don't handle being broken as well as some other bones in body might. One of these bones, called the scaphoid, is commonly broken during falls onto the palm of an out-strecthed hand. I don't care how experienced you are, it is a natural reaction when falling forward (i.e. if your board slips out from underneath you) to put your arm out to break your fall. Usually that means you fall onto the palm of your out-strethed hand.
Anyway, when you break this bone it is at a high risk of developing something called avascular necrosis - which basically means it is going to rot away and die no matter what surgery you have or how long you are in a cast. If you value moving your hand around, you don't want that to happen...
Probably more detail than necessary, but the important point with this is that if you are going to bother getting wrist guards they should have the brace on the top and bottom of the wrist - if anything the top is more important to prevent the injury described above.
I wear the triple 8 under cover wrist guards
- they are about $20, generally comfortable, and fit into most gloves without much trouble. They may not really serve any purpose most of the time when I'm riding, but I'd rather that than have a random bad fall and end up getting stuck with only using my left hand for the rest of my life.