That's a very big and sufficient use case, significant enough to warrant making this "concealment card" the norm unless there is a reason not to. Not the kind of thing you want a customer realizing after the fact.
I think you lose a lot of context. The screen shots help you remember what you're doing. In most cases, I would rather see the last state my app was in.
You're missing the point. Several, actually. Sometimes screenshots are not acceptable - such as when they show Social Security Numbers, bank account numbers and contents, or NSFW stuff (imagine switching when AirPlay-projecting for a group during a meeting). Sometimes screenshots hang on for a long time in the switcher - the session may have expired, but the thumbnail is still there days later, showing wrong context. Sometimes the snapshot is just plain unreadable because it's scaled down, making it unusable even if you want to see it. The whole point of the parent's source code IS to provide app state info WITHOUT just snapshotting the screen or showing a bland logo.
I'm just not able to see the point. You're the user and the device is in your possession. If that's not the case, that's what a PIN or TouchID addresses.
A OS X desktop analogy would be having sensitive information in a Safari window, then hitting the Mission Control key, and expecting the zoomed out image to be replaced with something obfuscating the sensitive information, only to be restored when you click on the window. I don't see what's gained.
Screenshots are fine as long as they reliably show the last thing the user saw before switching from the app. It's up to the user to shut down the app or ensure it's not in a sensitive state if they need to. If your browser is resting on a page with your social security number on it, then that's the problem right there --- not that there also happens to be a screenshot of this on the switcher.
Some niche apps might benefit from employing a scheme like this to bolster security (I'm thinking for example a password manager which requires its own passcode whenever you switch to it), but the potential minor security drawback for these niche situations is definitely not sufficient to require this across all apps.
Screenshots with sensitive info are not fine, that screenshot is stored on disk unencrypted. They also often outlive the running instance of the app. This bypasses any protection regularly used by the app for that information, and therefore the best practice is to blank out anything the typical user deems sensitive.