Asamoah is absolutely worth a shot as a LB. The only defensive questions I'd have are on crosses, whether he can always cover off surging attackers. So far at Juve, I haven't seen him make a mistake in that area. He's always in excellent position in his own box on aerial balls but he also does lack height so he could be climbed over in nervy situations.
Then again, there are/were quite a few great fullbacks who were short. lahm, evra, alaba, alves off the top of my head.
Then you think of the benefits. Well, you don't have to spend 10-15M on a replacement who might not even be an upgrade. You already know Kwadwo is firmly integrated and settled into the team. Chemistry is there already. And a LB usually has more time to cross since he's not the one driving forward with the ball, he's supporting. So you could even see an improvement in his attacking quality.
To the question about if I'd be comfortable with him marking Robben, Di Maria, Ronaldo, Bale, Ribery, etc. Well, there are only 2 players I'd be more comfortable with in space against those players - Barzagli and Vidal. Asamoah has shown he has incredible pace, agility, acceleration, strength and most importantly disciplined timing.
When it comes down to it, there are very few names available that seem like a certain upgrade on him. I mean, Schmelzer, Santon, Silva, etc. cannot be better than him IMO.