In my opinion, for most teams outside the top 6 in this modern game, fullback is one of 2 most important positions next to hooker.Edwards is good value in the salary cap if you have a 6, 7 and 9 like the Panthers do. He isn't a top fullback tho. Most of the best players in the game are fullbacks. For me fullback is the 3rd most important position and there's no way it's 4th.
When you get to the top 6, it probably drops to 3rd as you need a very good half to be a premiership contender.
This isnt a blanket rule however. For example... Roosters, Souths and Manly the fullback is the most influential and best player. They have good halves but the fullback is a standout.
The fact that NSW origin team has 3 fullbacks and 1 or 2 others in consideration for the other back positions (Gutho, Papenhuyzen) pretty much tells you everything.