Decentralized mining is key
Bitcoin's reputation as a decentralized system depends upon keeping the mining fairly decentralized. Although it is highly unlikely that a miner will act maliciously in the event of obtaining the majority of the network hash power. However, it is better for everyone involved that no single miner reaches this threshold. A decentralized mining system gives a lot of participants in the Bitcoin experiment peace of mind. After all, there is some serious money at stake now. Bitcoin's market cap, as of early 2016, is around 7 billion US dollars.