Here's a reason why I prefer Firefly to the P4 renderer.

This is the exact same prop (Stonemason's Hellcolumn 3). Firefly allows displacement (actually changing the shape of an object with a texture map) as opposed to bump mapping which pretends to add bumps like the P4 renderer does.

True, I don't always use displacement, but I do prefer the Firefly look to the P4 look.