I've dipped into the Left Behind series to see what the fuss is about, but am not interested in reading it in any detail for, it would seem, much the same reasons as you. Putting aside whether one believes in the infallibility of the bible as a religious document or not, is seems to me that the theology on which the Left Behind series is based is flawed.
Having said that, in the normal course of events, I'm perfectly happy to suspend disbelief to enter into a fantasy world - so I think the problem I have with the Left Behind series is that I'm not clear that the authors are intending to represent it as fantasy or whether they're pushing a more loaded agenda.
For me, the key to a book, whether set in a fantasy/future world or a current one is whether the world makes sense - I can suspend disbelief for the purposes of entering the world of the book but I cannot suspend it for plot elements that don't make sense within the context of that world. For example it drives me absolutely loopy in the fourth Harry Potter book that the whole story is built on the premise of a contest that Harry is entered into and has to win in order to get into the center of a maze in order to be teleported out of the school grounds to face Voldemort - when there were countless easier ways that he could have been whisked from the premises by Voldemort's right hand man disguised as a trusted teacher! As for the time traveling device given to Hermione in book 3 - the plot inconsistencies that that opens up are endless - not least, if she, a 13 year old, has her hands on such a device, how come the Ministry of Magic aren't using similar devices to leap back and forth in time to observe what actually happened at key events, even if they're not allowed to change them. Oops, I digress!
Anyway, back to the topic in hand - in short, I think with so many books out there you're not being narrow minded in choosing not to read the Left Behind series and, frankly, considering you'd probably only get into a disagreement with your friend about them if you did, it's probably prudent of you not to!