While it is not unusual for players to occasionally sit in on negotiations, the presence of Manning was a surprise and is an early indication that the union is successfully getting the support of its superstar quarterbacks. During the 1987 players strike and subsequent labor strife, several of the league's star passers broke union ranks by signing licensing rights with the NFL which weakened the union revenue pool.
Add Tom Brady into the Pro-Union group, which he has very much been so far, and the league's three most visible and most popular QBs give the Union the leverage they need to get a fair deal done with the league. This is a much better position than 1987 for the Union, as the league has to listen when these three QBs say something. Without them, the NFL isn't worth nearly as much as they are with Manning, Brady, and Brees.