Bitmain's contribution to p2pool will not help regular miners. They see that being proponents of p2pool will raise their status in the eyes of potential customers. However it appears they simply want to run their own solo mining operation using their own p2pool instance which is really of no relevance to miners. Even if they dump their hashrate directly into regular p2pool, it just increases share difficulty even further, making it even harder for regular miners. The only way they may be contributing is by adding code/improvements to the p2pool network, but in my experience every hardware manufacturer directs the absolute minimum amount of effort towards software/drivers as possible to just expedite delivery of their hardware.

Offtopic about ckpool: There are public pools using the ckpool code - regular, solo and testnet. Just check the announce thread for the listed pools.