From my own experiences, I used to never have forward lean. Rode just fine for 3-4 seasons without any complaints. But this season I started playing around with the lean one notch at a time. Every time I push it a bit forward I notice dramatic improvements to my board performance.
I can carve harder, feel more stable at higher speeds, and turn a lot quicker than before. It does burn out my legs a lot faster than before but over time I got used to it. Without forward lean I struggle a little going down moguls and black runs but with the proper lean I'm able to ride through them with a certain... fluidity and style o.O