I don't think you have to have a FP for FotLK, but I got one for my last trip. I liked not having to worry about getting in. The FP+ guests were allowed in first, but that's about the only major benefit, I can think of at this point.
For the parades (FoF, MSEP), I also don't think the FP+ is critical, but it was nice to have. You still can't wait until the last minute to arrive and plan on getting a curb/rope spot, but the general area seemed far less busy than other areas. So in that way, it was very nice.