Sandbagging as a "successful" Cat 5 is just silly IMHO, but who are they really hurting? Taking away other Cat 5's chance to win a race with no prizes?
Personally, I can kinda see why someone might sandbag as a Cat 3. Moving from 2 to 3 in many areas means you're only going to be doing P,1,2 races, and the races are going to be significantly longer. Racing Cat 2, IMHO is a significantly bigger committment than racing as a 3. You could rationally decide you still want to race, but don't want to step up to the demands of racing P,1,2.
Fortunately as a pack fodder Cat 3, I don't have this moral dillema.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.