Well the point of it for them as a business is leverage and a hedge. They get to leverage and buy more equipment at the cost of users taking a portion of the earnings...and hedging since the resale value of equipment (GPUs and ASIC) reduces eventually.

There's no way they would have gotten that big without funding from investors and users...it's just a mathematical game of scale, which is even more apparent since the the founders are really math geeks.

As for users, it really doesn't favor anyone using the service since you give up so much power...but there is opportunity none the less...and users basically take a chance for the opportunity.