A better analogy would be using the bittorrent protocol to download different files. Each of the different p2pool forks represents a different share chain, a different set of users, and a different history.

If you switch share chains, you don't "lose" your work. If a block is found on the chain you left within ~3 days of you leaving, you get paid for the work you did on the old chain. The math works out such that if two p2pool chains are equally efficient, then no matter which chain you mine on, the expected value of your payout remains the same regardless of how much time you spend on each chain and how often you switch between chains.

(Of course, the two chains are not equally efficient. jtoomimnet has about 2x higher transaction fees, so your expected revenue will be higher on jtoomimnet than on mainnet. Recently, jtoomimnet's expected revenue has been about 10% higher than mainnet.)