I would say an outright no no, (when racing at least) and IMHO!
Here's why, the little time you may gain in speed from the kicked up rudder will be lost ten fold when tacking or jibing due to having the additional process of buggering about with the rudder, setting down.
This can be harder than you may think when under pressure and coming into a turn at speed as the rudder will want to ride high and will take some force to lock down.
Having said that it can look good while flying a hull to impress the girls
Depends on what level you want to take your sailing.If you want to compete at an elite level ie Worlds ,North Americans then it is worth practicing and perfecting.Like Matt said it forces you to steer near perfectly.By near perfect I mean very little rudder movement and not pinching.
Cat sailing upwind usually involves long stretches on one tack,over that long stretch perfect steering will pay dividends.
Really the only time you could get hurt is on shifty lake sailing.
Then again if you are just a Weekend type racer you are probably better off sailing more conservatively with both rudders.This is not a technique you just want to experiment with on the race course under pressure.You want to have it perfected by hrs of practice first.
Matt ,like myself have seen the good guys first hand,they don,t do it just to look cool.But I must admit it does have a cool look.