The art forms all have a symbiotic relationship. Sometimes it results in disaster (take the musical version of Gone With the Wind, which was savaged by critics), sometimes it produces classic pieces of art (the film of Gone With The Wind).
The aim of adapting novels is not necessarily to improve them- whenever that is the aim, the film normally fails (take the 2009 film of Brideshead Revisited). Film versions of Anna Karenina will never be able to do full justice to the novel- there's simply so much in it- but they can be good films in their own right. I'm looking forward to it (Tom Stoppard's doing the screenplay) but the casting of this 2012 film is dubious and I don't entirely trust Joe Wright, considering that he destroyed Pride and Prejudice.
EDIT: I think the link I originally gave leads to the message boards so here's the main page:
http://www.imdb.com/title/tt1781769/