It's an interesting point, and I do feel there's some merit to it. Why did I play so much World of Warcraft? I assure you it wasn't the gameplay. It was fun building a character, but really what I was there for was being able to spend time with friends who live far away, and because I wanted to see the world. It was a cool world with tons of stuff in it, and that's a really big draw. I took thousands of screenshots (same thing with Oblivion -- I have a whole huge photo album of like 1800 shots or something).
That said, I don't agree that the two should be separate. I feel like they should be mingled, which to me is exactly what Aquaria, like many good action/adventure games before it, finds the perfect balance of. It isn't hard as nails, but it's a far cry from "easy", and that makes everything -- and I mean everything from the emotional impact of the story to feelings of achievement to the discovery of new environments and creatures -- so much more meaningful. If you could just swim there, see it, and leave, what would be the point? Challenge, in my opinion, is an inherent facet of what drives the emotional experience in a game. It involves you and stresses you and makes you work for it, which is exactly the thing that books and movies can never do, hence never personally investing you in nearly the same fashion.
That said, I think an easier mode for this would be pretty great. As I've stated, my wife just adores everything from the art to the story and characters, and she's hooked just watching me play, which again lends credence to seeing being a big part of the experience, but she still really wants to play and it frustrates her that she can't because she knows the game will be too difficult for her. But the main experience for the average player should never be compromised, so IMO games that are built to be too easy so that they can accommodate more players will always be gaming failures. If you want to add modes for those who want super-duper hard play and for those who want easier play because they just don't have the coordination or lengthy history of playing games that the rest of us do, that's great! Just make sure you first aim for the best overall experience for the average player (which I think Aquaria does perfectly, as I said).
So yeah, this has gotten lengthier than I intended, but it's an interesting topic. To summarize - I think challenge is an inherent part of games, but if people can build multiple tiers of that challenge for those who simply can't hack it, that's a pretty cool thing. But the base experience needs to be the primary focus, and if push comes to shove and making an "easy" mode is going to somehow detract from balancing the primary mode properly, it should be canned or released in a patch or something instead of wasting development time on it.