Typically ranged classes beck-pedal, because they try to increase distance while melee tries to close distance. So.. The 3 biggest ones would be:
1. Speed. Doesn't matter if opponent has gap closer or not. Moving away faster is always better than moving away slower.
2. Vulnerability. Exposing your back to opponents who have more powerful abilities when positioned behind you.
3. There's no reason to beck pedal ever. You can do everything you can while strafing that you can do while backpedaling. Some players think their abilities won't work unless facing the player, when in fact, you can throw attacks over your shoulder just as easily while running away. And some consider strafing as only moving to the side, when in fact you can strafe directly backwards with some mouse adjustment and take advantage of everything from above.
Players who argue that backpeddling is fine simply haven't mastered the movement of their character yet. Or find it awkward to hold the right mouse button down and maintain control over the camera.
Players who backpedal are by no means bad players. Some great players even claim to backpedal in order to draw fire or fake being a noob. I think that's bs personally but whatever.