If the purpose of a Wikipedia or wiki in general is to communicate information... then creating a wiki to serve a small group of people who are already fluent in other major languages does not serve the purpose. I wouldn't object to a wiki of a type "Wikilang" if the idea is only to save the crucial and various aspects of the language itself for historical purposes in a way not conducive to a Wikipedia article, but a full Wikipedia or other project seems unnecessary.
On the other hand, the question should be asked: are we saving significant resources by denying the creation of dead or constructed language projects? Would the folks creating these projects otherwise be contributing more constructively, and do these projects require significant meta-resources if the local communities remain small and isolated? Particularly in the areas of PR risk and legal liability for projects that are WMF-hosted, the answer could easily be that significant resources are theoretically devoted to each project. In that case, it makes sense to continue to deny these projects.
Nathan