You don't have to stay still to stay in the sweet spot.  There's a range you can move with that, and I have not had trouble staying in the sweet spot in cars/buses/etc.  What IS trouble is the gyroscope on vehicles.  

Also, people carrying around glasses tend to always be wearing them.  If you're always wearing 3D glasses when you don't need them (or already have glasses) it can be a bit of trouble.  This is especially true when you think about the fact that 3D glasses have tints to them, making it harder to see the things around you.

It does, and with 4K on the horizon it will become more common...but still rediculously expensive. 

lol I didn't say all the time. Who doesn't have a backpack/purse/briefcase/bag/pocket/etc. to carry stuff in? Afterall where's that 3DS going. Surely you can't hold it all the time. My parents don't wear their reading glasses all the time. An old friend who should have been wearing his glasses more often than not rarely did, only kept them on his person. I mean, come on, surely people are capable of carrying glasses with them like they do other things.

I was exaggerating a bit on the whole standing still part, but it's definitely true that movement will throw the 3D out of whack, screwing up your gameplaying having to readjust. Staying peferctly still would be your best bet to avoid having such issues, but of course that's not possible.

People may have stuff to carry things in, but #1 having an extra thing to carry around with your 3DS is a hassle, #2 it is more time consuming to have to always try to wear the glasses when you want to see the 3D #3 you would still have some sort of slider or button to activate the 3D, otherwise your screen would be one big mess if you didn't want to see the 3D #4 it would suck if you broke your glasses and had to replace them.  You can be careful and still have them break because of the fact that they are separate from the system.  Someone else could easily break them.  

Overall, it's just very inconvenient, and I move around constantly with my 3DS with 3D on and it's just fine.  Also it really is an issue on games where every moment of the game is important to know what's going on.  Luigi's Mansion doesn't suffer from that, so it really is a selective issue.

#1 and #4 apply to anything, including the 3DS so these are kinda mute points.

#2 - Come ons, it takes like 10-30 seconds to pull out your glasses and put them on. You can't be that lazy. :P

#3 - Yes it would still have a slider. What's bad about that?

If it would really cause one so many problems, simply leave the glasses at home, and turn the 3D off. You couldn't have cared that much about the 3D if you're not willing to take some glasses with you. While I care about 3D, I care about being able to play more, so I've no choice but to leave the slider barely up just for that extra vibrance. A much bigger inconvenience it is, having to get interrupted in the middle of a game I'd say.

a full room.  it is using very very different technology than the 3DS.  it bascially stacks 3 HD tvs one on top of the next so that the pixals physically are in 3 dimentional (even if the spacing is only millimeters apart) and then manipulate the output of each to give a greater sense of depth than was really exists.  so full 3D and no "sweet spot" which is pretty much a requirement for any reasonable consumer product.  4k is needed because without it you can't even get 480p resolutions of your content.

..but it is still really expensive right now.