Coordination and utilization of the proper musculature surrounding the shoulder complex are necessary to prevent injury to structures like the rotator cuff, biceps tendon, and labrum. When a climber rests in a "dead hang" position the scapula elevates and abducts while the glenohumeral joint is tractioned superiorly. This motion may decrease the subacromial space while the load of climbing only further adds to the stress on the structures within the shoulder.
Being able to appropriately coordinate the lower trapezius (LT), upper trapezius (UT), and serratus anterior (SA) is important for scapular stability. Activation ratios are a good way to compare the relative contribution of individual muscles. Patients with subacromial pain syndrome have been found to have a disruption in the coordination between these muscles. UT/LT ratio was higher, and LT/SA was lower in patients with subacromial pain syndrome, alluding to the importance of strengthening the lower trapezius for injury prevention.
Citation: Michener (2016) "Relative scapular muscle activity ratios are altered in subacromial pain syndrome"