How to keep PowerPoint from mirroring my monitors when switching apps?
If you switch PowerPoint to display mode first, then switch to a mac app (using command-tab), then turn off mirroring, the setting should then stick. Otherwise, as you found out, Microsoft is sure nobody but powerpoint needs display mode.

In case some reader doesn't know how to turn off mirroring: If you go into "System Preferences/Display Preferences" Arrangement tab (only present when connected to a second display), uncheck the box that says "Mirror Displays".

Also, on the bottom of the Display Preferences pane, you can check "Show displays in menu bar" to access this setting more easily without going into preferences.