There obviously seem to be some arguments for and against exposing “jumpers”, and various different reasons and types of “jumpers”, and as the left alliance, you don’t always know the reasons why one left you.
One very important point, IMHO, for defining what is a “jumper” at all, is the quick repetition of the join-leave circle. Switching alliance once in a game-life, or once every 3 months does not seem like especially “evil” or “punishable” behaviour to me, neither does temporarily visiting other alliances when communicated to and agreed on by everyone involved.
So, when talking about “exposing jumpers” or anything similar, I think it is important to only talk about those who jump often and frequently, and don’t have any strong reasons (like language barriers and searching for proper alliance to chat in their native language) for it, but only do it for abusing others to gain personal advantages.
And then, again, it’s hard to know the reasons, and of course it is lots of work to basically observe if any of those who left you does then quickly join and leave several other alliances, to see if they really jump frequently, or just did it once for whatever reason. And then ask yourself, is it worth constantly tracking all those who left you for two weeks or so, only to see if they really are “jumpers” in the more narrow sense? I think not.
Wrong accusations aren’t any useful either, for many obvious reasons.
So, if you really want to publicly expose jumpers, then you really need to be careful whom to expose and about whether the accusation is actually true, basically limiting this to a very low number of notorious jumpers, which then might have very limited practical use in total.
Summing up: There might be a few players who deserve being mentioned/exposed, but generally I don’t think that’s the way to go on a bigger scale, hence I wouldn’t advocate for putting lots of effort into a listing and monitoring system for “jumpers”.