Easy for me to convince you! We'd only done Fantasmic with the package. This year, I suppose due to free dining, we couldn't book a package and decided to go without reserved seating. WDW recently changed the EMH evening for MGM and we weren't aware of it. We showed up for Fantasmic and, surprise, it wasn't showing. To make up for our inconvenience, Guest Services gave us reserved seating for another night. We showed up and sat in great seats. The area is on the side, but actually makes for better viewing of the film clips shown on the spraying water. I don't think the center section would see these clips near as well as we did. We were right at the front as well. And, there was enough room to be comfortable.
As I looked over at the folks being squeezed into the non-reserved areas of the stadium (remember it was at least 90 degrees and everyone was sweating like pigs), I commented to others in my group, "Don't ever let me try to do this without reserved seating. If I even suggest it, just shoot me." They wholeheartedly agreed.
Frankly, I'm not a real Fantasmic fan. We did it about 5 years ago and now won't bother again for at least another 5. But, when we do, it will be with reserved seating or not at all.