First of all, elbows out (like bodybuilders also recommend) is at a mechanically disadvantageous position.
The further the perpendicular distance between your elbow and the side of your body, the greater the resultant upward force must be at the shoulder joint due to mechanical moments. This alone places unnecessary stress on your rotator cuff.
Secondly, the shoulder is not spherically symmetrical - It is designed to rotate forward more easily than it can rotate backwards. The should joint is a "ball and socket joint", but it functions best in the plane of motion parallel to the side of your body.
Test this yourself. While standing, take your arms, bend your elbows 90 degrees. Bring your elbow in to your side. Pull your elbow back, as if performing a close-grip barbell row. You should be able to raise your upper arm behind you such that it is nearly parallel to the ground.
Now, put both your arms out in the position you would perform a wide grip, elbows out bench press - upper arms raised at 90 degrees to your torso, and parallel with the ground. Pull your arms back in this plane of motion as if you were lowering a barbell to your chest. Notice that your upper arms do not go as far back as they did with your elbows tucked by your sides - there is something hard in the way at the back of your shoulder. This is your rotator cuff coming into contact with the acronym, which is the rear roof-like part of your scapula.
When you bench press in this way, your shoulder is not free to rotate backwards thoroughly. Tension is shifted off the muscles and onto the acronym and weak rotator cuff.
