Here's the reason I think it's unnecessary: for me, at least, and for some but not all others, it contributes less than nothing to the experience.
A few years ago Jeffrey Katzenberg stood before us at a NATO meeting in Hollywood and talked for a long time about how important and new and revolutionary 3-D was. This was the future, he said, and he rolled out the ol' comparison to sound and color. He said it would make films more powerful, engaging, emotional experiences, even in retrofit: he said specifically that, among a few other titles I don't remember, The Godfather and A Man For All Seasons would benefit and become better films by being (again, retrofitted) in 3-D. I was like, this guy has no idea, which seems silly because, you know, he's Jeffrey Katzenberg. But he really doesn't. Later on when new and old movies were receiving the ret-con 3-D and bombing in the box office, he talked about how they weren't doing it right, were lazy with the process, etc. Finally he came out and said he feels 3-D treatment in post would kill the new tech and advised against it. In interviews he never talked about the quality of the movies using the tech, just the tech itself and whether he felt it was properly utilized (mostly according to this metric, as far as I can tell: if it made good money, it was good work).
Something that's really hard to do because we can't un-see the seen and un-know the known and then after contrast independent, personal, fresh experiences, is to watch a movie in both formats. The best we're able to do is watch one and then the other and try to give each a fair shake. And I feel I've done this with a few titles, sometimes with the 3-D version first and others the normal, and I've come to the conclusion: 3-D means fuck-all for how much I actually like a thing. Again aside from the technical annoyances, I found Avatar no less visually splendorous in two dimensions after having seen it in three. Toy Story 3 was not more funny or poignant or wonderful or visually stimulating because its second review had an extra 'D'. The vertigo-inducing sequences in Up were vertigo-inducing in either format. And so on.
When a movie is done well enough to actually draw you in, the format becomes invisible. Great story doesn't need it, great acting doesn't care. And as a visual matter 3-D is unnecessary because our minds normally infer three-dimensionality. We don't need it to build out our sense of space, because we already recognize space in the film. It's useful in those moments where something's flying at the screen, but I don't know anyone sees these moments as better than cheap gimmicks...that tend to pull one out of the experience for an Oh right, I'm watching a movie moment, literally interrupting engagement with the material. So why do we need it?
NOW the technical problems come in. If I can watch great film with or without, why am I paying more and wearing these obnoxious glasses? If the action is mind-boggling in either case, why am I watching this dimmer version that has obvious judder during movement? If most of the features being released look awful and are poor movies besides, why do I want to keep dropping my money on them, sending the signal to studios that this is something I want more of? Higher frame rates and other tweaks may eliminate some of the image problems, but that still leaves us paying more to wear glasses for something utterly divorced from the actual quality of the feature. And even if Prometheus turns out to be the best argument yet for 3-D (I guess it's possible), supporting it here still only plays into the studio/exhibitor impulse that audiences want 3-D, beyond consideration of movie quality (trust me: I'm nearly blind from face-palming at their e-mails).
So I say it's unnecessary. So I say plenty of the time it's actually detrimental. I find most of the enthusiasm even now comes from the angle of what how it might be used in the future, the possibility that the 3-D grass will be greener. There's a lot of "Well, Avatar was pretty neat, but I'm looking forward to some great film artist getting his or her hands dirty finding out what this thing can really do." And I'm saying: I don't think so.
Hey, if it works for you, great. But I'm still calling it unnecessary.
Well, I'd take Cameron's enthusiasm with a bale of salt. Avatar didn't exactly have much meat on its bones, and Cameron's got a big vested financial interest in the success of 3-D: the Cameron Pace Group (a company he co-founded with Vincent Pace) makes its money selling 3-D equipment and software, and he's more or less going for broke on it with his future projects (Avatar 2, 3, and possibly 4)....or James Cameron's passion.
EDIT: David Bordwell made a great blog post a few days ago covering a few of the same points, including the bit about the Cameron Pace Group. Bordwell's a reliably excellent writer on film (Ebert, for one, worships the guy; the blog, co-authored with Kristin Thompson, is called 'Observations on Film Art'). I came to discover his work through this excellent piece examining a shot from There Will Be Blood. Anyway, I thought I'd share:
Originally Posted by http://www.davidbordwell.net/blog/2012/04/22/its-good-to-be-the-king-of-the-world