I keep wifi passphrases in a location that is just as secure as the ethernet network to which the wifi is tied: on a piece of tape on the bottom of the WAP. Looking at this again, in the unlikely event I forget the passphrase, seems not to be a burden.
I wouldn't recommend that in any circumstance, it's completely false security really.
MAC filtering is incredibly easy to bypass, all Malory has to do is wait for another device to connect and clone their hardware's address.
It's false security because (especially for wifi) MAC filtering is like a verbal password that's been written down on a billboard. Everyone can read it so its not a secret.
If it can be exploited, it should be protected as much as it can be.
We all saw what happened with Google's StreetView cars capturing the local Wi-Fi traffic. The same thing could be used by somebody hacking into Google's servers, downloading the Wi-Fi passwords and any possible maps data if possible and workaround getting into people's Wi-Fi networks. If it is not likely but it is not impossible.
Or if you're law enforcement, the knowledge of the existence of the resource is enough.
From a governmental law enforcement perspective, that's an incredibly valuable trove of data. Instant network backdoors, likely with no physical entry required.
Even if you're not using the data to sniff networks, you can use it to build detailed relationship graphs between people and places.
Every few months someone rediscovers that Google also syncs Wifi credentials between devices (perhaps when logging into a new device and finding it tethers itself nicely to the network on its own).
It's a matter of convenience, Wifi passwords are only applicable at a certain range, and other restrictions could be applied even then (like hardware whitelisting etc) it's not your bank credentials.
Here's a reddit discussion (from 2 years ago!) http://www.reddit.com/r/Android/comments/g6ctt/just_upgraded...