Let's look at this from another point of view
Zerg is nice and healty for server
Ally#1 is stronger -> Ally#2 wants to compete so they recruit more ppl -> Ally#2 has more ppl and they stronger-> Ally#1 wants to catch up and they also invite ppl
Randoms, new parties are welcomed in top allies
More ppl in top allies = more pvp = more fun
The only problem is lags during siege