Comment 2 for bug 1335568

This is basically a feature, not a bug. system-image-cli is a developer tool, as is writable mode. Developers are supposed to know what they're doing, so s-i-cli never changes its behavior based on writable mode and in fact never looks at /userdata/.writable_image.
You would have to pass `--build 0` to force a full update.

In IRC discussions, Sergio mentioned that it worked differently in Saucy, where if writable mode was set, no updates would happen via the system-settings ui. If that behavior's changed, then it's a ui bug. Note that system-image-dbus also does not look at /userdata/.writable_image and does not change behavior based on writable mode, however s-i-dbus is purely triggered through D-Bus activation, which typically is only invoked by the systems-setting ui.