It's not really clear how this works from their site but I think they're basically just sending your raw blend files to random users without any protection or guarantee, legal or technical, that either the blends or the renders will be deleted afterwards. That's going to go down well with my clients. "What the prototype renders were leaked by some random teenager from Birmingham? No, I've no idea how they got hold of them. Honest guv!"

Ah well time to add another clause the contract I use when subcontracting, something like "I affirm that I will not make use of Rentaflop during the fulfilment of this contract" should do it.