Its not he network that needs them, the network is already storing them. What about decoding that MD that your computer needs to process? Or the string of MDs that is storing that important data.
Its that the client is using the keys to en/decode a lot of things. And the volume of data to be decoded could be large.
The hardware wallet was designed and good for signing transactions for blockchain transactions and the computer passes that onto the blockchain network. This is a great use case. The hardware wallet is processing very small amounts of information and small (cpu, memory) powered devices that can fit in a hardware wallet can do this easily.
Now for SAFE the keys are used to encode and decode up to a megabyte of data at once. Hardware wallets are not really up to that task. They don’t have enough internal ram for one and not enough processing grunt. SAFE is not using keys like a blockchain wallet is using keys and therein lies the problems for a hardware style wallet.
Now we need an alternative system that can keep your keys safe while they are being used.
Maybe a secure USB device that is read only (with a secure write enable ) and contains an APP (or actual OS that boots from the USB).
We are still at the point were we need the power of a decent computer with RAM to do the en/decryption of MDs etc.
Now if there was a specific use case for this safe hardware “wallet” like countersigning SAFEcoin transactions then that would be good. It uses your credentials and wallet ID’s keys to send the transaction to the network might be a viable use case for a hardware style wallet