I don't think it's really important whether or not we need it as such, I think it's more that the people who go to the pictures to be entertained for an afternoon - the majority - want to see a film in 3D, and therefore it's only right that it should happen.
As far as the visuals go, Christmas Carol was amazing anyway, and the 3D only improved this by giving it literally more depth, and Zemekis uses this to great effect. There are some scenes in it which are frankly beautiful, and this is something which I found in Up. Both films had breathtaking scenery and lighting effects, and in 3D these were only improved upon, though perhaps Christmas Carol uses it a bit better, but then Zemekis has been working with 3D for a lot longer than Pixar (though I am not saying the 3D in Up was poor at all - far from it).
I haven't seen Coraline or My Bloody Valentine, so I couldn't say whether or not it was improved in 3D or not, but I don't think it's as simple as a yes or no answer to any of your questions. It has its good and bad points, and in some films the good outwieghs the bad points of the 3D, and with the improving technology including Motion Capture, there are going to be less and less problems with aching eyes and headaches.
From an artistic p.o.v., I would say that 3D is definately worthwhile, since it really heightens emotive scenes, as in Christmas Carol. In fact, this film is an indicator that Zemekis might have actually found a style to base his characters on, much like a more detailed version of Pixar's style, which hopefully means he's decided to stop going for realism in his characters, though the CGI backgrounds are pretty stunning.
Whether it will improve old films is a good question. The trailer for Toy Story 2 3D looked pretty impressive, and surprisingly not pointy-visionish. But I think it will depend from film to film. Those that are visually good will benefit, but at the same time only if they're done well. Those that aren't nice to look at will be pretty pointless in 3D, and will gain essentially no benefit other than a re-release and some more money (which I think is inevitable). Unfortunately I can see films like saw going 3D, "for an even more sadistic experience". This is clearly not a good thing since excessive violence is in most cases pointless anyway, so making it 3D is only making it even worse. But I could easily imagine Jurassic Park or Star Wars for example in 3D, and this would look very good. But then you wonder if something like NCFOM would benefit much, because while there are lots of nice vistas in it, there are few amazing effects which would be stunning in 3D, so there needs to be a balance made because 3D like anything else could spiral out of control.