Another side of the debate that hasn't been looked at here, is this:
Poly strings compliment a *High Spin* playstyle, more than anything. Players using extreme grips and hitting with big topspin are *brushing* over the ball, rather than the classic playstyle that worked better with Gut and dense patterns; i.e. hitting flat and *through* the ball.
One stroke is going to transmit much more shock back to your arm than the other (the flat stroke that pummels the ball head-on). Players with a playstyle of the Borg/McEnroe era and even Agassi/Sampras era are *typically* going to be seen using Gut, gut hybrids, syn gut, or multis. They are typically going to be using heavier sticks and smaller headsizes, hitting the ball flat, or with "standard" spin with a continental or eastern grip and finishing points at the net when they can, generally speaking.
Players of the modern playstyle, looking to maximize spin with their western or extreme western grips and grind out the baseline, are going to gravitate toward the lighter, stiffer frames, with poly or poly hybrids. This is just a matter of preference as certain equipment caters to certain type of play and mechanics. The extreme topspin stoke, the reverse forehand, etc; employ more of a 'brush up' on the ball, rather than a flat, follow-through into the ball. The brushing up onto the ball is naturally transmitting less shock into the racquet and down to the arm.
The latest pro string logs show that even the pros are stringing their poly in the 50s (in most cases), or below. The ones still stringing over 60 are few and far between. There are some clay court specialists stringing below 40lbs. It is also known that gut at 60+ tension still retains a high level or elasticity and resiliency, while co-poly strings reach their maximum stretch point much earlier on and do not absorb shock as well as gut.
So again back to my point, is that poly is going to be more detrimental to one's joints/tendons/etc if you are not brushing up on the ball, as the classic continental or eastern grip type of stroke will naturally transmit more shock at impact than a high-spin stroke which transmits less shock down through the frame into your arm.
While I do agree on the one hand that one's racquet or string can impact the arm health of the player, we cannot completely point to ONE THING ONLY, i.e. 'the string killed your arm', or 'that racquet is arm friendly', etc etc.
Form, mechanics, equipment... all these things are but pieces of the entire puzzle and each one plays its role. Certain playstyles will benefit from certain choices of equipment. Other playstyles have no business using certain equipment. Stringing up co-poly at 70 pounds or for someone 'looking for a durable string' are just BAD IDEAS. Stringing up full gut for a baseline grinder who hits full spin on both wings using a Wilson Steam 16x15 is probably also a recipe for a different kind of disaster.
Head Pro Tour 280/630. Tour Bite 18/Mosquito Bite. Sampras serve, Agassi backhand and Federer forehand. heh heh