Applications could be exposing intents they assume will be used by trustworthy applications (i.e. apps in the Play Store). A user could download a Droidscript (which as I understand doesn't trigger the unknown sources policy) which then tries to use intents it shouldn't need without asking the user for permission.

If Droidscript required unknown sources to do anything (not just APK exports), then other apps could check the unknown sources policy on the device and disable certain intents (which they may do anyway at the moment, since that would mean that the applications installed may be untrustworthy). But this way there isn't any way to tell.