Lots of pros string poly and poly hybrids over 50 lbs and a few string them over 60 lbs. Blake, DelPo, Agassi, Cilic, Isner among many over 60 lbs with poly or poly hybrid. Nadal, Davydenko, Djoko and many others in the 50-60 lb range. Federer, Hingis, Jack Sock, Max Mirni, Filippo Volandri among a bunch below 50 lbs.
Trend seems to be moving down in last couple of years.