I agree 100%, lietkynes, and what you've described is one of my favorite aspects of the game. The fact that you can run through, jump in, and manipulate these unwell minds underscores how we all share the basic human problems, and what we define as 'insanity' isn't so abnormal after all. For instance, I felt uncomfortably sympathetic to Gloria (and I think many writers, actors or other creative types would feel the same way). Wandering around in her head made me realise that a) she's more logical and rational than she looks and b) I'm less logical and rational than I look. But without the freedom to manipulate Gloria's mind through Raz, I still would have been an outsider.
Or take someone like Milla, who is ostensibly a sane and rational character. But even she has her demons and nightmares, and she spends considerable effort reshaping the landscape of her mind to mask them. (In fact, upon replaying it, I think her level was one of the most disturbing ones in the game; I swear I heard the nightmare children's voices remixed into the party music). But you never would have gotten to see past her apparent sanity without manipulating her mental world.
Anyway, I think one of the geniuses of the game is that it highlights how we're all a little crazy inside - it's just a matter of degree. The visuals are just one way (one major way) that it does that. The exploratory game mechanics that you mentioned are another. It all works together, I think.