I believe Nintendo should have given us the option to use 3D glasses for the 3DS, if not make it mandatory to have glasses in order to view the 3D. While glasses free 3D is a really cool concept, in practice it's more trouble than it's worth, atleast from my experience. While I'm playing a game like Mario Kart 7, I can't afford to have the screen suddenly blur up on me, then frantically have to adjust my position back into that perfect spot while I'm trying to focus on driving. Seeing how much I like 3D, it pains me having to put the slider to just barely on, and only cause it makes games look more vibrant. Even in less hetic games, it's still annoying having to stop playing every so often to get it back into that sweet spot, hence the 3D is usually left just barely on. Surely I'm not the only one who feels this way right?

After having my 3DS for nearly two years now, I can easily find the sweet spot with little trouble. Of course the argument can be made that many people have trouble finding it and something like this shouldn't be so difficult to do, but let's look at using 3D glasses. Is it really better? Having to carry around 3D glasses with your handheld device makes it much less portable. They could easily break while the glasses-free 3D screen prevents those issues.

Also, I hear the 3DS XL has an easier-to-find sweet spot.

People already carry around glasses, so this should be no different. They wont get broken as you'd put them in a case, which if small enough, can fit in your pocket. At worst you can just wear them. Definitely better than having to find and stay in that sweet spot, which if you're on the go like in a bus or car, will be uber difficult. I can find the sweet spot easily, but it's annoying having to do so in the first place and even more annoying having to stay completely still, otherwise you have to disengage yourself from the game to go back to that sweet spot.

On the 3DS XL there seems to be less trouble with the sweet spot. Once you have the technology behind it implemented in a real TV it shouldn't be much of a problem anymore.

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.

I've never understood all the complaints about 3D on the 3DS. First it was people complaining that stuff wasn't jumping OUT at them, even though Nintendo's ad-slogan was always "Take a look INside." Then you have people complaining about it being harmful for young children or people getting headaches...well, newsflash, there's a SLIDER. You can keep the 3D off at all times. It's non-invasive. Those who don't want it, NEVER have to use it, and yet it's a nice bonus for people who DO like it and want it. For me personally, I've played almost all of my 650+ hours of 3DS with the 3D turned up to the max. I love the depth it adds to most games and to me it adds to the immersion and presentation. I also find it very easy to find the sweet spot. The only time I didn't like the 3D from the 3DS was when I first got the system. For the first hour or so, my eyes just weren't used to the sensation and so it was a tad uncomfortable, but I finished the Deku Tree in Zelda with it on. The next day, I picked it up and turned it on and haven't had a problem since. Once my eyes adjusted and expected it, it's been great. And yes, to me Glasses Free 3D IS worth the trouble. I'd NEVER use the 3D if I had to wear stupid special glasses just to see it.

Wear 3D glasses everywhere I take my 3DS?  No thanks.  I like the glasses free 3D personally.

I think a full-size TV with glasses-free 3D does exist.  Not entirely sure, though. 

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