Tim,
I don't agree or disagree with the notion. The notion being does framing or defining the shot make it easier. Defition for the sake of defintion is a concept that most on this site will say they don't like as a tool for the golf architect, and for the most part I have to agree. The argument must be looked at both psychologically - how what the golfer sees affects the way they feel about the shot - and physically - how what the golfer knows is actually there affects the shot. Lack of definition will typically make it more difficult for a player to visualize the shot and then commit to the physical act. However, if what helps to define the shot is a severe penalty, it can work as an even a greater force to either making it difficult to see an aggressive shot or to commit to hitting such a shot. Such will also vary greatly with the level of the player and the type of shot the player is comfortable hitting. For instance, if a pin is tucked to the left side of a green with water down the entire left side, a player who is comfortable controlling a draw is more likely to see and execute a shot at the flag, while a player who is more comfortable fading the ball will have more difficulty finding the nerve to start it on a line that will result in a short putt. However, if the hazard to the left is a deep bunker and the fader is a great bunker player, they might not have as difficult a time taking an aggressive line.
Have you ever heard a touring pro say a shot, a hole, or a course fits their eye? This is what they are talking about. Courses that regularly give the golfer nothing to use to help them fit their eye can present subtle challenges.
Also consider Pete Dye's golf courses. They look very difficult, but he has oftened used hazards and features to help the player see the shot. Just on looks, Pete's courses should be the hardest in the world for the best players, but good players often score very well on his courses, if they know how to read what he is offering and use visual aides to help them see the shot better.
Does lack of definition inherently make a shot more difficult? It depends on what the defining element is and what paradigm the player brings to the shot.