again, look at how many transactions are with satoshidice, compared to p2pool, p2pool is like 0.1% or even less...
There is a key difference between SatoshiDice, which is an application of bitcoin that pays transaction fees, and p2pool, which is a miner that is paid transaction fees and generates coins.
When bitcoin hits mainstream, the block reward will be small and mining profit comes from transaction fees. Transaction fees are partially based on size already, and in the future I believe that it will increasingly depend on size.
if you don't care about more regular payouts, then solo mining is still better for the network (both in terms of security and blockchain size)