Yes, it's meant for writing, because all, even the heaviest the read-only testing can be safely done on a private mainnet nodes.
It will be up as long as we need it, during a heavy development times such as before the HardFork we might want to re-initialize it more frequently when we introduce breaking changes, but it's meant to live in parallel to mainnet and always initial state of the mirror will be very similar to mainnet (i.e. every restart).