Depends on the kind of gameplay mechanics you're after.
My limited experience of peaceful was mostly due to being a total noob, so it almost served as a tutorial mode so I could at least grasp the basic game mechanics before I re-added them.
This would just expand on that a little more- allowing a gentler introduction to combat, and the experience of fighting the boss mobs ect...
You can get knocked into lava on peaceful mode anyway (overly friendly wolves)- but if monsters aren't causing physical damage one would assume theres no knockback from the attack- so no risk of being pushed in.