Frozen Chosen, Left Behind

I'd be happy to help you out if you will provide me with some of the specific arguments they are making. The premillennial camp is not unified, so it is hard to guess precisly what they might have said to you.
I haven't read the Left Behind series, but I'm familiar with typical forms of Dispensational premillennialism, which is the basis for those books. In my estimation, Dispensational premillennialism is a false system based on poor exegesis. I would classify the doctrine and its assertions regarding the end times as "fantasy."

A good book showing a crucial flaw in the Dispensational premillennial system is Dispensationalism - Rightly Dividing the People of God? by Keith Mathison. In this book, Keith demonstrates that the Dispensational system and eschatology is based on the fundamental error that they mistake God as having two separate people with two separate plans. It is basically a question of ecclesiology: Who are the people of God? The Bible teaches that God has one people, visibly manifested within Israel in the Old Testament and the church in the New Testament. The Dispensationalists believe that the church and Israel are distinct. The future "kingdom" or "millennium" they imagine is intended as the time when God deals with Israel again. But God is not going to deal with Israel again, at least not as a separate entity from the church.

I should add just as a personal anecdote that I was raised as a Dispensational premillennialist. I read The Late, Great Planet Earth and believed it because it was exciting. But I didn't understand it. Even then I thought its interpretations were so far out that one had to be a genius to understand them. I just couldn't see how any of that stuff really came from the Bible. But I believed it because my church taught it to me, and I respected my teachers. I thought Dispensational premillennialism was a universally accepted truth. When I read the Bible, I thought it said something completely different, but I chalked this up to my own lack of insight. Years later, when I met someone who was not a Dispensationalist or a premillennialist, I was shocked to hear such heresy expressed by someone else for the first time. He didn't tell me what he believed, but asked me what I thought the Bible taught. When I explained to him what I thought I read in Scripture, he said, "Well, that would make you an amillennialist." Thus, I wasn't raised on amillenialism; I just learned it from the Bible. ;-)