I agree with @JLB_ep on this issue. I personally believe that seeing a win rate will be used by some people to make hasty decisions and I believe this unfairly hurts the alliances who have lower rates, however I also think that players should be allowed to see the number of flags per war used. For example players a and b are both looking to join alliance x. Player a is hardcore and constantly uses all of his or her flags, while player b is more relaxed using around 4 to 5 flags per war when they can. If alliance x has a participation rate shown of 98% assuming occasional real life commitments that curb the players ability that would be an environment more suited to player a while player b would probably prefer to search for a different group of like minded players. Different people different preferences, but they each are better off finding a right alliance for them and alliances are one of the few things that keep a significant number of players, myself included, returning even after other disillusionment occurs.
I hope to hear other thoughts on this, but personally I think this is a good compromise that allows for the alliance to grow and still act as a unit.