Personally I prefer email for this reason. Too often I've seen coworkers who become "the guy who helps". The guy doesn't say no, who answers questions that have nothing to do with their job. I like to be helpful but I don't want to be the one who fixes everyone else's problems to the detriment of getting my own work done. It's easier to say "you should ask Joe, he's in charge of that process" in an email, for me anyways.