I posted this earlier but here it is again:

• Transaction Fees given to Miners as a REWARD
• Instant payout when a block is found
• No coins stored on pool server (dependent on pool operators configuration)
• Maintain your PPLNS level switching between connected p2pools
• No account registration or passwords to remember

Added to that list that it is much harder for the pool operator to cheat miners.

Also,

• No single point of failure in the pool (when p2pools are connected)
• There are small statistical advantages increasing income vs traditional pools

Read more here:
https://bitcointalk.org/index.php?topic=153232.0;all