If second citizenship is allowed in their EU home country - good for them.
Other then that, how does it help taxwise? Residency in a country with territorial taxation would do the same like their CBI passport. And once the EU country of which they still carry a passport comes up with citizenship based taxation their CBI passport of Nevis will not help much (rest︀ assured the EU will pressure all these CBI countries to tax these people).
So, from︁ my point of view CBI is outdated. Naturalization is the way to go if one︂ is serious. Anyway, in a few years we will know more ....