Ok, I made this one up. While a properly fitted pince-nez provides a very secure fit, I do not recommend that you play rugby while wearing them. Unless of course you are very fast and never get tackled.
I do have an interesting bit of historical trivia. During WWI, the U.S. Army advocated the use of pince-nez due to its comfort while wearing a gas mask. Pince-nez: strong enough for war but not for rugby!