I've never liked the "if you travel through time you can't change anything" argument because it's no fun at all. It doesn't allow for anything to happen. I think that if you want to have a good time travel story you need to have some other kind of reasoning.

Which is why I was so glad to find the site I'm linking to at the end of this post. It describes four main time travel plot types. Obviously, these aren't the only types there could be, but the site does a good job of not only explaining different possibilities but also of showing that people who argue that time travel MUST happen a certain way and that any movie or book or story or TV show that shows different must be WRONG are basically being stubbornly and sillily (there is no way that's a real word) closed-minded. My favorite time travel type is Type Three.

-Eric Sleator
Mon 12 Nov A.D. 2001