Supposedly, acquiring a strong track record may make an individual identity eligible for bonuses and higher-value tasks, but my experience with the task source isn't long enough to even know what that would look like, to a CoinWorker.
Could a malicious individual enter somebody else's BTC address and screw up their accuracy/reputation/whatever?
Only if the individual knew which BTC address was being used for CoinWorker. I have a separate address for everything I use, so there is no way that anybody could know my address for CoinWorker unless they had access to my wallet. And if somebody has access to your wallet, your CoinWorker rep shouldn't be your top concern.