They are trained sometimes. The problem is that in tournaments, wristlocks can be somewhat of a grey area. Depending on the rules, they may or may not be legal. Most people avoid them in tournaments because they don't want to get disqualified for pulling what may be an illegal move.
Roy Dean is a great resource on wristlocks in BJJ. He is a black belt in Aikido and BJJ, and he has a DVD out about blending the two together with an emphasis on wristlocking from standing and on the ground:
"Let your food be your medicine, and your medicine be your food"
They can often be great for making another submission more effective, say if someone is really fighting a pressing armlock. In the tournaments I do you can use them, I believe at either beginner or intermediate and up.
Member of DaJoGen MMA school under Dave Hagen and Team Chaos fight team under Denver Mangiyatan and Chris Toquero, ran out of Zanshin Martial Arts in Salem Oregon: http://www.zanshinarts.org/Home.aspx,