I stopped mining awhile ago, but I've left my p2pool node up. It's averaged 36 peers in the last year and it's bitcoind node is well connected. It has a high speed internet connection and has had excellent uptime.
Is a node like mine actually helpful for p2pool? I'm assuming that having a well connected node is good for spreading transactions and protecting p2pool from some kind of network attack. Am I right, or is a non-mining node not helpful? It's just a git pull and a restart ever now and then, so it's not much effort.
If a non-mining node is helpful, is there anything else I can do to make it more useful? Maybe set the max connections to something a lot higher so the pool sees more transactions? I'm just guessing.
More importantly, your node will help with propagating blocks found by p2pool users. The more pervasive the p2pool network is, the fewer blocks are likely to get orphaned. forrestv apparently has something in the p2pool code that distributes the block super fast to prevent as many orphans, but I think that more nodes will also help to do the job too.
p2pool is unique in this respect, I don't think any other pool has a dedicated network of relay nodes running code to help propagate blocks, the centralised pools probably have to keep an eye on who their bitcoind nodes are connecting to and make sure they are using the most reliable nodes, the most well placed and those with the most capacious bandwidth.