If the Earth stops turning, you wouldn't have the majority of the Earth's surface plunged into darkness. You also wouldn't prevent the eclipse from being transitory--it's the Earth's orbit around the Sun (and the Moon's orbit about the Earth) that cause the eclipse to come to an end. All the bodies involved move out of alignment.
To say that Malekith means to make the eclipse permanent is problematic. The movement of these bodies is a consequence of their relative positions and their gravitational pull on one another. As CherokeeSam pointed out, they did filming with the helicopter over the Thames on October 21, when the tide would be high. Tides are caused by the Moon's gravitational pull on the Earth. So on the one hand, we want to include the real effect of gravity, yet on the other hand, we want to override that through magic. It seems like we can't have it both ways. And science fiction/fantasy generally works better if they minimize the number of scientific laws they bend or break. That way, regular science can lend elements of verisimilitude to the story.
If we're to have an eclipse, it seems more likely that it's just to give the Dark Elves freedom of movement, as you suggested earlier. It could also be a timing thing, as CherokeeSam suggested in the main spoilers thread. My one caution about that would be to be careful it doesn't wind up evoking the plot of The Dark Crystal: "When single shines the triple sun/What was sundered and undone/Shall be whole, the two made one/By Gelfling hand or else by none."
Another idea with eclipses is to cause general panic on Earth. We know enough about the movements in our solar system that we can predict these eclipses well in advance (I have friends who are planning a trip to where the eclipse will be in 2017, for example). So if one were to occur that we did *not* predict, that might be more frighting for our modern society than they would have been back in the day.
Isaac Asimov had a short story, "Nightfall", about a planet with six suns that generally does not know darkness. Once in a thousand years, night falls, and the people panic, riot, and set their civilization back, well, 1000 years. Scientists on the eve of sundown are trying to preserve enough scientific knowledge to allow future generations to break this cycle. http://en.wikipedia.org/wiki/Nightfa...y_and_novel%29
Hmmm, that has parallels to the Ragnarok.
But it may be that there is no actual eclipse in this film, just metaphorical ones.