The only thing is if everyone is invited, what year would it end up being in? Did anyone compare the distances between the most convenient Grand Ball location and the outermost regions to medieval forms of travel? How long would it take for the messengers to get the invitation out?
Some people have already used only one action per alliance rather than one action per member affiliated with the alliance. Would this be the same concept? (Was that allowed? I hope so. Spending 3 actions on 1 alliance seemed ridiculous.)
Also, it means something happens other than the trade/marriage/train troops/form alliance -fest which has been (quite understandably) the first round.