Since both the hapless Esperanto and the hopeless Interlingua already have an established place, there is little viable argument against conlangs in general. The only reasonable option is to limit them to the very few that promise some utility.
But why should that only apply to conlangs? Aren't Elsatian, Manx or Bislama also vanity projects? But no one questions there right to exist. Klingon might well be less of a vanity project than these languages, so why treat them differently, if your only argument is the utility of a project? I don't think we should disallow Elsatian or Manx, but rather allow Klingon as well as Tokipona. If some people feel unhappy about having inter-language links for these, I would suggest that one should be able to switch conlang-links on or off by user preference, if this is technically possible.
Marcos.