Cuddlydemon said:
In and of itself they aren't gimmicks, no, but the vast majority of games where either or both show up, they are used as an afterthought and really don't add anything, nor would playing the game without either option subtract anything.
The problem as I see it is that there's still currently no good, practical, cheap way of making them work properly (counterexample: dual shock - a couple of miniature analogue joysticks (proven, reliable, cheap technology) and small motors with different sized cam-weights (ditto) located respectively in the natural resting position of the player's thumbs (practical) and in the controller "wings" where the vibration can be directly transmitted to their palms (ditto).
So far it seems for motion and 3D you can have two of those features at best, but typically only one. Until that happens, you can't have games where it's forced as the only controller option for a wide variety of games (as dualshocks rapidly became with later PSX games, and were the default for PS2, for example) - and I don't just mean the showcase launch titles (Wii Sports, Kinectimals etc). And until THAT happens, the feature will only ever be a gimmicky add-on, because you have to make the core game compatible with falling back to 2D and/or regular button-bashing / stick pushing controls.
Generally speaking, I view 3D as something I shouldn't expect anything from, so I can be forever pleasantly surprised when it's actually used to enhance the experience. This has thus far only happened in movies, and only when implemented by Pixar.
I have to take a polar opposite stance. If it's in there, I want it to be used. At the very least in a clever, subtle way to enhance a feeling of "being there"; better, to actually be a proper part of the game mechanic somehow, e.g. having to shoot/avoid things that appear at certain distances but may be at entirely different scales or decorated in otherwise illusory colours & patterns, so the usual 2D depth queueing tactics we've become unwittingly used to in games (distant things are darker/greyer, any particular item only comes in one discrete size, shadows are always cast regardless of the lighting if they're necessary to locate something as being at a particular point on the ground if it's capable of flight or otherwise ambiguous) just wouldn't work and binocular vision has to take charge.
And along with that I must say Toy Story 3 was an utter let down in terms of both immersive 3Dness and doing anything "spectacular" with it... in fact it felt flat a lot of the time. I could watch it in 2D and not have lost anything from the deal... unlike the presumably much cheaper (and less-experienced-team) Despicable Me which would lose more than just its obvious 3D-jokes in the credits from being depth flattened.
Tron Legacy was a bit naff in that regard too (ignoring its other, catastrophic failure to have a second act between the first and third), vs Avatar (and Star Trek, too) being excellent spokesmovies for the effect being worth the eyestrain. Maybe it's just a Disney-conglomerate thing? They've done excellent 2D for so long, they just can't think in three dimensions any more.
The example 3DS games I had a tinker with on a local chain store's display were sort of halfway between: the effect was palpable and impressive, but the game didn't seem to be compromised in any way by flicking the switch to "3D OFF".