The reason they made it optional is because games that don't use it at all can use the consoles horsepower to look better. Also it has motion controls and using that with 3D wouldn't work. Doesn't solidify it as a gimmick at all. That's like saying anything on any console that doesn't force itself on all games is a gimmick which is ridiculous thinking.
Not "anything on any console", the single biggest selling point of the system. The kind of lethargy surrounding statements like "we're not going to make games specifically for this feature", especially for Nintendo, doesn't bode well. Even with the Wii, which they at least went to some lengths to make games specifically surrounding motion controls, third parties couldn't be arsed to make a good game with good motion controls because Nintendo couldn't be arsed to do it themselves. Even their best games either used them very lightly or not at all.
I still firmly stand by my "gimmick" assessment. For this specific implementation of the 3D, there's nothing that leads me to believe that it will be used for anything but cherry-on-top eyecandy.
Sorry, but this argument is entirely inconsistent if you play any other console. I can't think of a single "selling-point" feature that is mandatory for any game on any system out today. Are 360 games required to use LIVE, a primary feature of the console? Of course not; not every game has multiplayer. Not every PS3 game is required to take full advantage of its Cell processor, which, if Sony is to be believed, is a major selling point of the console. We can only presume that not every NGP game is going to be required to push its hardware to the max, arguably the major reason to buy the handheld.
And you know what? Who cares if it's eye candy? That's the entire freakin' point: to look pretty and wow consumers. I can't speak for most people, but I don't expect 3D graphics to revolutionize anything, but rather to augment an enjoyable gaming experience that little bit further. And there is nothing wrong with that. "Gimmick" is just a buzzword hardcore gamers use to make themselves feel less insecure over different demographics enjoying things like motion controls, dancing games, fake-guitarathons, and "casual" smartphone games.