Isn't the "finish" of the turn the phase at which most of the energy is loaded into the system? This is the point at which the snow, your skis, and your legs are all compressing; storing the work that gravity does upon you as potential energy. This energy then gets 'unleashed' (I enjoy thinking of it like this) by forcing your body laterally into the next turn. ZOOOM
If you don't finish your turns, you are just wiggling down the hill. This limits your vertical speed control, as you aren't converting the work done by gravity upon you into lateral momentum. Thus, you have less control, less fun (arguably), and don't look nearly as cool (which is also essential). To control speed with this methodology, you are forced to perform 'stivots', 'slarves', or whatever other word people have come up with for pseudo hockey stops. These require more quad and calf tension than carving and put the skier off balance, as the center of mass is diagonally above the skis.