The Effect of Posterosuperior Rotator Cuff Tears and Biceps Loading on Glenohumeral Translation- Su, Budoff et al/ Arthroscopy May '10.
The long biceps tendon is one of the shoulder's backup systems to control GH instability. Tendinitis is usually a warning that the tendon is becoming a primary stabilizer.
A pretty simple study; they cut fibers of the subscapularis, supraspinatus, and infraspinatus to different degrees and combinations. Then mesured the amount of anterior/superior translation of the humeral head in the glenoid with and without biceps loading. The long head biceps tendon reduced humeral head translation up to 53%!.
Relocating the tendon from the scapula to the humerus is often a surgical treatment that increases shoulder instability; which is a precoursor to rotator cuff tendinitis.