Storage rent, like incentivising relays as is the primary goal with Pocket, is essential for the sustainability of a blockchain. Without it, you get a tragedy of the commons, where storage is overutilized due to everyone free-riding it as a free public good, and consequently it becomes bloated, it then takes longer and longer to sync the chain, even as a warp sync, fast sync, etc. At the moment in Ethereum there are probably only a few archival nodes that store the whole state. While it is true that there probably will not be as much state to store in Pocket, as it is not a Turing complete smart contract generalised transaction ledger like Ethereum, there will be state to store: e.g. batches of relays, staking/unstaking, validators joining/leaving, etc.
- a suboptimal way with gas: http://gastoken.io/