I've noticed that this topic is discussed ad nauseum over at a certain doomsday obsession message forum. I have a theory.
Most of the people who believe they will be raptured in their lifetime are incredibly, incredibly conservative in their Christianity. They are Bible literalists, live a very legalistic life, believe that the end is so close that there is no need to think about the future, etc. As much as I hate to say this, because I don't like painting with a broad brush, many uber-conservative Christians tend to be incredibly discriminatory in their behavior. They don't like people outside of their chosen denomination. They don't care to interact with followers of a different denomination (or religion outside of Christianity). They prefer to stick with their own kind and wallow in a state of self-pity and borderline hatred for anyone with a differing view of life. They need that sense of camaraderie because their beliefs and actions are skewed so far to the extreme end of fundamentalism and Bible literalism that they find it difficult to fit in even among other Christians.
Anyway, to get to my point here. The fear of interfaithism, the emerging church, social tolerance, etc. stems from the fear of looking bad in front of others. What happens when the majority of Christians begin to open their minds and their hearts to others, instead of keeping Christianity an exclusive club? It magnifies their intolerance and loathing of others, which makes them look like the bad guys and they are so weak in their faith that other people's perception and support of them is paramount even if they won't admit it. They want to be able to sit in the seat of judgment, but they don't want to be judged.