I think that they gave you a bs line, cuz that is part of the safety system. If the rear brakes rupture a hose, your pedal will fall to near the floor driving the rear-most piston mechanically into the front-most piston. Then the two of them will continue to slide down the bore until the front brakes stop the car. This might pretty much use up all the piston travel. The frontmost piston has a stopper on it that prevents the frontmost seal from bottoming out. So on a new/rebuilt unit, bottoming out the pistons is no biggie........ plus that is how we bench bleed them. Now if this happens to you on a MC with many years of service on it, it could be that the end of the bore has rust in it; (not likely cuz it's uphill and water don't collect up there, but let's say it did), then the piston seal could run over the rust and not appreciate that very much. But that is a reeeeeeeeeally long stretch. MCs are really simple devices, and if the fluid is kept uncontaminated, should last for decades.