From the whitepaper:

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Eliminating “abuse” is not possible and shouldn’t be the goal. Even those who are attempting to “abuse” the system are still doing work. Any compensation they get for their successful attempts at abuse or collusion is at least as valuable for the purpose of distributing the currency as the make-work system employed by traditional Bitcoin mining or the collusive mining done via mining pools. All that is necessary is to ensure that abuse isn’t so rampant that it undermines the incentive to do real work in support of the community and its currency.
The goal of building a community currency is to get more “crabs in the bucket”. Going to extreme measures to eliminate all abuse is like attempting to put a lid on the bucket to prevent a few crabs from escaping and comes at the expense of making it harder to add new crabs to the bucket. It is sufficient to make the walls slippery and give the other crabs sufficient power to prevent others from escaping.
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More from the whitepaper:

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The impact of this voting and payout distribution is to offer large bounties for good content while still rewarding smaller players for their long-tail contribution.
The economic effect of this is similar to a lottery where people over-estimate their probability of getting votes and thus do more work than the expected value of their reward and thereby maximize the total amount of work performed in service of the community. The fact that everyone “wins something” plays on the same psychology that casinos use to keep people 
gambling. In other words, small rewards help reinforce the idea that it is possible to earn bigger rewards.
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