Arm and shoulder issues have been around since the dawn of the game. It doesn't get much more "arm-friendly" than a 14 oz Kramer or Maxply, with a flex of 40 (I'm guessing here) and strung with nat. gut, and yet folks came down with tendonitis et al. all of the time playing those sticks - myself being one of the legions of folks to search for more forgiveness in a lighter, more manageable package - ergo the prince pro, then graphite, and so on. So I think it's an issue of mechanics, prerequisite strength, sensible use, and adequate rest. Pitchers (and catchers) come down with pitcher's elbow, and they're not striking anything. So I think the stiffness issue is overrated when it comes to elbow issues honestly. I know others have had different experiences, but to me, it's an occupational hazard of the game, not so much of a specific type of racquet. The pros hit as many balls in a couple of weeks as most rec folks hit in a year or more - with pretty stiff set-ups; but then Jimmy Tennis drags his flabby ***** out to hit after watching the US Open, buys a Bab because Rafa uses it, hits with the crummiest technique imaginable, shanks 36 balls in a row, and then proclaims (to the internet, since nobody else will listen), "This Bab is SHITE! - it ruined my arm!!"
Nosiree, I think the Babos and others are ok - it's more the technique and relative fitness of its user that matters. Inadequate musculature = strained tendons, plain and simple.