The motives of the aliens in WotW don't make much sense. However, they aren't outside space and time, like the diuternal entities that populate the stories of H.P. Lovecraft, whose mere dreams cause catastophe and dissolution. Spielberg's aliens are (just as H.G. Wells' were) quite physical, and subject to the laws that govern flesh, those of gravity, of time and space, and last but not least, of Murphy.
Why? Aye, there's the rub. Must they be simply evil? Or does humanity's presence in the universe pose some sort of implicit threat? Obviously, life does--they are (as in H.G. Wells) susceptible to microorganisms. They catch cold and die. I'm not spoiling here--this is exactly the same ending as in Wells' novel and the 1953 version. But what if, what if the Aliens were being damaged by television? No, wait, before you scoff, what if radio waves, going out into space, were lethal to the aliens? Reruns of Three's Company actually killed them? They have to destroy humanity in order to preserve their own species? Or at least, destroy television?
Another thing about the film vs. the novel. When H.G. Wells published WotW, Louis Pasteur had only recently revolutionized science and the world with his discovery of microorganisms. A remake of this story should find something, anything, that is as recent as Pasteur's science was to Wells. I would think that nanotechnology would have been a likely choice. There is something wonderful about Wells' ending, anticlimactic though it is--human beings have the right to be here. But it is better to put more science into science fiction.
If movies are feeling the pinch, and they are, box office is extremely poor these days, it's because Television is catching up. Shows like Six Feet Under, Deadwood, Nip/Tuck, The Shield/Rescue Me, Oz all succeed because they place character at a higher premium than spectacle. As spectacle comes more and more from the palatte of a software program, the days of justifying $20 million to sign a movie star are numbered.
The entertainment industry is on the cusp of vast change, due in part to satellite, cable, HD TV and DVD. We no longer have to go to a cineplex in order to see something thrilling. For too long movies have been spectacular but devoid of heart and soul. That's going to change.