I know this is an old post but I want to add something for the viewers.
I agree with the answers about the cleaning and repolishing and damaged marble floors. Damage may accur because of high traffic or lower quality polishing by the stone's producer. In this case it seem like it is about high traffic.
You can do celaning-repolishing but it seem like you will have the same problem in the future. It is not you are doing something wrong but probably your stone is too soft to use in a high traffic area.
If it is, I can suugest you to consider honing your floors rather than repolishing it. You can hone all the floor for once and than you can use some enhancer (must be applicable for floors) to catch shiny look.
Honed surface will look like a whole, the areas you use more and the tiles on the sides. I have done this couple of times before and it works very well.
Of course there will be a diferent look. But it saves you spending the same cost again.
I disagree about the one answer, which is "do not use the marble"
Usually natural stones are durable and requiress less maintenance. It doesn't mean everyone will have problem with their marble floors because some people had. It is a matter of taste. I suggest you to pick right material for the area you want to install. Get some more information before you make your purchase.