Optimum crew awareness and coordination is one of the main advantages of a side-by-side cockpit. It's the reason Gen. Curtis LeMay had Boeing change the cockpit layout of the B-52 from the B-47-style tandem design used on the first two B-52s to a side-by-side configuration. And it's the main reason Boeing decided to use a "conventional" dual-yoke system on the fly-by-wire 777. With fly-by-wire, of course, you can use any kind of control input device you want.
When one control yoke is moved by a crew member, the other yoke moves as well. If the autopilot is flying the plane, both yokes will reflect the autopilot's control inputs. Even if the flight crew is busy with charts or messing with the radios, the movement of the yokes in front of them will provide a continuous visual indication of what the autopilot is doing. If the captain is flying manually and the first officer suddenly has to take over or become involved in the control of the airplane, he probably will already have a sense of what's going on control-wise from the movements the yoke has been making in front of him. It's an awareness thing, and can be a critical factor in an emergency situation.
Sidesticks do not provide this level of crew awareness. The movement of one stick does not move the other one, hence the non-flying crewmember will not be aware of the other person's, or the autopilot's, control inputs other than by instrument indications and by what the airplane is actually doing. Even if the sidesticks did move together, their location puts them outside the "awareness bubble" of a non-flying crewmember if he's involved with instruments or controls in the center of the panel or on the aisle stand. (Sidesticks could be made to move together, but this would involve yet another complicated system on an already complicated airplane. Yokes are tied together by a simple, proven, mechanical linkage.)
Pilots who fly sidesticks think they're the greatest thing since sliced bread, and pilots who fly yokes say there's no other way to go. In fact, both systems are excellent methods of connecting a pilot to an airplane. The floatplanes I fly all have yokes, but the few hours I spent years ago in a Citabria and a Schweitzer 2-32 sailplane convinced me that a stick is a more "natural" way of controlling an airplane. But after interviewing and involving hundreds of line pilots from around the world, the decision was made to use a pair of "conventional" yokes on the 777 for the human engineering reasons I've outlined above.
Incidentally, the same philosophy extends to the throttles. When the engines on a Boeing plane are being controlled by the auto-throttle, the power levers on the aisle stand move to reflect the action of the auto-throttle. The auto-throttle on an Airbus does not move the powerlevers. They simply remain in the last place they were positioned before the auto-throttle took over. It's that awareness thing again. If the flight control computers determine a power application is needed during some phase of the flight, Boeing pilots will see their power levers move up the quadrant. The only indication Airbus pilots will have is the acceleration of the airplane, possibly the sound of the engines spooling up, and the readings of the engine instruments. In a high-stress, high-distraaction environment, on final in severe turbulence, for example,these are all things that could be overlooked for several critical moments. That's not to say it's impossible for Boeing pilots to overlook the action of an auto-throttle, but at least they've got some big levers moving under their noses to indicate what's going on.