It would show weakness. Is GOB such a miserable place that you're at risk of losing members because someone sent a message? That would be quite sad. Any alliance that has its IA in order should honestly thank other parties for stripping away disloyal members and dead weight. Needless to say, the act of "poaching" in this sort of mass-messaging manner is pretty stupid for the "poacher," as they would get only bottom-of-the-barrel members for their efforts.
Yes, "poaching" is often used as a paper thin excuse to start a war, when one can't find any better reason to start a war they want, but it isn't a real CB, it simply isn't. It also indicates that an alliance views it's members as property instead of free individuals. It should be no surprise that the origins of this "CB" come from off-world, from none other than NPO, when they needed an excuse to hit someone, and also because they're a literal cult.
It's also usually rolled out when one individual makes a free choice to join their friends in another alliance. For that reason alone, the norm of using "poaching" as a CB should be done away with. People should be free to make that choice, and bad, whiny leaders need to take the message that they're a shit leader if they can't retain people.