Are we coming back full circle? 20 years ago all engine mods were on the same lobby client on the same server - until they became clients (with an -s) and then at some point servers (with an -s).
I would have two comments on the idea:
a) The lobby of ZK is not perfect in terms of features (queues never work, some issues with voting maps, etc.). Nothing serious, but I would rather see lobby improvements than BA games.
b) I think the playing styles of ZK and BA are different enough that I would rather prefer players to find and stick to their preferred style, rather than playing "more or less the same" in both BA and ZK. I guess there is a subset of people that could play both reasonably, but not the largest set.
There should be though more cross-advertisement of the games existence, especially for unhappy/bored players. Because if someone stops playing one of the games, he might as well try the other one.