Direct-to-instacash miners benefit is 0-fee (the block solver gets a slightly higher reward, but technically that evens out over the long term), better orphan rates, so more money in your pocket, and provably in your pocket.

Depending on how long you want to be a miner, it also is basically the only pool that can't steal your money, the only pool which can't take your hashrate and do something else with it without you knowing, the only pool that, if 100% of all miners used it would NOT create a centralized hashrate problem, one of the ONLY pools to payout in actual coinbase (Eligius does as well) and ..  well honestly there's basically no point in mining elsewhere than P2Pool or Eligius.