It's a good thing you're Phil O'Sophical, 'cos you certainly aren't Si N. Tific. '3D' and 'stereoscopic' are, for all intents and purposes, the same. You give each eye a different image; your eyes see them and perceive objects / aliens / battle axes / whatever. Your brain cares not a whit whether it's looking at a physical or holographic in your room, or at two displays.
Parallax is determined by head tracking, which you can do. But, in general, you sit fairly still while playing games or watching movies - if you're in a movie theater moving around enough to generate any serious parallax between on-sceen objects, you're going to get tossed out on your ass.
It's not a 'single special effect' any more than color is; it can, as color can, be done badly or well, be added clumsily or done correctly, be implemented and shown well or poorly. The real-world performance of any of those things has no bearing on the technology itself.
And it's not 'easy to do with a computer' - I'm pretty sure about that even though I'm not sure what you mean. If you mean post-converting, it's incredibly difficult. You get crap results, but it's still a herculean effort for the tech guys to achieve them.
As I've written above, I'm not sure that 3D adds anything or CAN add anything to film. It seems to me that it would break too many odd side-effects of film to be worthwhile. But I'm sure that plenty of people thought that way about color - which was indeed pretty bad in many cases. Hell, when they shot The Wizard Of Oz in color back in the '30s, the film used for color was so slow that the lights alone heated the sets to 105f+. If you did it wrong, you got really bad results.
The industry's tragedy-of-the-commons response to Avatar's success has undoubtedly hurt 3D, but I don't think it reflects on the intrinsic values (or deficiencies) of the idea.
I *can* say that TV-level implementations vary in their success. Some of them make everything look like sparkly snow; some of them darken things a lot; some have incredibly sensitive glasses that lose sync if you bobble your head a bit. The Sony I've used extensively looks wonderful, with shockingly low light loss - nearly too good to be true. With gaming, however, that experience comes at the cost of setting up the *software viewpoints, FOV, and convergence* correctly. Do it wrong and you get brain-inverting psychedelics that a first-timer will think, "3D, but it's terrible!" about. Do it carefully and you think, "Jesus Christ on a pogo stick, I'm *there*!"
The reality is far more complex than "The industry sucks and this is bad like a toy."