I'm surprised by such conflicting information in this thread! I personally have no experience with either the dinner package or using FP for Fantasmic, so unfortunately I don't have much to add.
However, as one who has read all of the pages involved, here are my observations...
1. Some people may or may not be confusing using FP seating with the dinner package seating.
2. It sounds like while FP doesn't start letting you in before the regular crowd, the fact that the FP line is much shorter means that people in the FP line would get in to get good seats well before most of the people in the stand-by line.
3. Citing the above, I get the impression that the dining package is the ONLY one that offers reserved seating, but that those in the FP line will get better seats than the majority of those in stand-by because of the fact they are in a shorter line.
4. I do not know how well they check that guests have dining package vouchers before allowing people into the reserved area, but perhaps some with FPs are going into the reserved dining package area thinking it is for those with FPs?
Just some thoughts to help try to clear up some possibilities of why some comments seem to contradict others...