What is the purpose of an app disallowing backups (android:allowBackup="false")?
You don't need root to backup data from an android device to a PC, you only need physical access and to enable debugging in the developer menu. This lets you copy data from the app's internal storage, which may contain sensitive information that you don't want to let somebody steal, like your facebook access token.

I agree that it is pretty trivial in most cases, but there are some cases which require this extra layer of protection against attacks where physical access is involved.

Note that in order to get around this you would need to root the phone, in which case the sensitive data in the internal storage would be wiped when the factory reset occurs.