The benefit of using an open toe foot is when FMQing you are able to see where you are going a lot better than if it is closed. On my Juki TL98q the foot came closed and my husband cut the foot open the the width I preferred and then sanded it so there were to rough edges. The closed foot blocked my vision and made it more difficult for me to know which direction I wanted to go to...to create a smooth flow.
The closed foot works better for the reasons listed by other members.