CI 3 native session driver?

What exactly do you need the memcached session driver to do which it doesn't already?

Memcached does the same thing CI3 does: it provides its own save handler for sessions. Both are equally "native" PHP sessions.

I already have that setup and it supports failover. If one memcached server goes down it can use the other one without losing session data. I couldn't seem to find this in CI's documentation besides providing weight values. With memcached you can write to more than one memcached server and when it fails it just starts using the ones that are available.