I don't think I'm understanding your argument. It seems like you think that the individual ATC decides when they have radar coverage and not. If he can't see you, he can't see you. Also, like someone else said, the whole point of instrument training is to be able to do EVERYTHING without ATC's help. We don't train radio out and full procedure approaches for nothing.

Also, there are ENDLESS aircraft incidents that have not been trained for. Have you ever practiced ditching? Have you practiced what happens when the top of your 737 rips off? Have you practiced when your 3rd engine blows up and severs all control to all 3 axis'? As instructors, we cannot start to imagine all the things that could possibly go wrong, however, we can train students in nearly improbable scenarios because it teaches decision making skills that will transfer to all emergency situations.