When I think of fire arrows, I always think about how quickly that arrow is going to be flying through the (usually cold) air. I can never wrap my head around how it would feasibly stay ignited... Even then, once it hits the cold hard floor or embeds in the ground it wouldn't be particularly terrifying and more of a slight inconvenience, but not anymore than tripping over arrow shafts. Such a small flame(considering it reaches it's target) would have tremendous trouble burning any of the clothing that the soldiery are wearing, anyways.
As for siege equipment, I would say that it is easy enough to clad a siege tower in protective materials. It's a different story if you cover it in oil, but that would require some degree of adjacency to the tower and at that rate you could just drop a torch.