Without a LONG story this ride and going back to last spring posts, was of great concern for me (terrified of heights and HATE Soarin') and my DS who is disabled with FoP having a setup that could trigger issues for him.
YES YES YES, they are important.
Your vehicle is right at the edge of a drop and could somewhat be prepared to Soarin' so you might say .... well a seat belt would do and the vehicles are side by side so it would take a great effort to climb over the front to fall off.
BUT the floor moves, a good bit. Your vehicle will tilt left, right and forward much more so than a Soarin' seat. Soarin' has a back, a large seat and arms on either side of you. This vehicle has no back, a small motorcycle type seat and no sides at all. If you were to get up, you would not be able to maintain balance because the floor is always moving, a lot, to stay in sync with the movie and for you to feel you are flying.
The biggest issue is the floor breaks away from the back of the room. As it moves there are a couple feet of gap between your floor and the back wall floor. This gap opens and closes throughout the ride. I watched all around the room my first ride just to take in the mechanics. Should someone get out, they'd likely fall as the floor moves, slide back towards the gap and without any more description it could be really really bad.
Disney can take no chances that someone would attempt to stand or get out of their seat. Unlike Soarin' where you are hanging and likely stay put, FoP there is that comfort that you are just "in a room". AND in front of you could be a 1, 2 or 3 story drop.