In plain view means just that, in plain view. Even using your analogy of headlights on a police cruiser, you still can't use headlights to peer into someone's house THROUGH THE WALL or ROOF.
The spirit of the law is that people have the right to do just about whatever they want in their house, behind closed doors/walls without being subject to a "casual inspection" by the police. Sure, it allows some people to do "bad things" without getting caught sometimes, but more importantly, it keeps the government from being able to micromanage your daily life.
Sure, because of "global terror" and a bunch of other scary words, people are more readily giving up their personal rights for "safety", but that doesn't automatically make it a good/smart thing.
Basically, if it isn't grossly obvious that you're doing something illegal, the Police should leave you the hell alone and go find someone who IS breaking the law in public. In my experience, that's not a very difficult thing to find...