Addendum to: http://www.sciforums.com/threads/brain-in-a-vat.158342/page-6#post-3420272
A standard for judging a possibility to be deeply useless might have to focus on the possibility's origins, and it having a substantive reason for why it would be of practical service to any group (as either regarding it as only potentially being the case or if cognitively holding it to literally be the case).
Dreaming is our first and inherent encounter with simulated reality, and thus it's arguably the inspiration for many if not most BIV type proposals throughout history (Zhuangzi's butterfly story directly recruited the oneiric medium). Such a category of possibilities also caters to a context which we're routinely familiar with in everyday life: "Am I being fooled or deceived by such and such?" Certain technology has also been influenced by that category, if its first developments were not outright triggered by fascination with it.
In contrast, an arbitrary concoction like "There is a cat-sized, iridescent gumbo monster in a lab chamber whenever no observations or verifications are being carried out" is just that: It's haphazardly invented and not significantly provoked by either circumstances native to our common experiences or existing traditions. As well as not stimulating the kind of interest and argued benefits that a non-facetious belief could be founded upon. And thus both deniers of it and agnostics about it also being engendered in reactionary response (i.e., the kind of possibility or belief which attracts a crowd). This however, does not exclude "cat-sized iridescent gumbo monster" having minor usefulness as a gimmick device in a particular instance of intellectual acrobatics. Perhaps only psychotic ramblings and gastrointestinal noises can be contended to be completely, philosophically useless.
As for possibilities like ghosts (the general idea of ghosts and not specific claims of a ghost which could be testable), when they are not banished by the presets of an ideological or methodological view (within the latter's jurisdiction)... They are apparently elements of historic and lingering societal customs. Which is to say, the proposal of ghosts is not random and spur of moment, without stimulus or motivation, without historical precedent. And even when they're misconceptions of observed events, the idea of them thereby nevertheless plays a role in some people's intermittent thoughts and experiences. In addition to the origins, "reasons" for belief have been built around them, albeit skeptics outside the subculture would dismiss such as unjustified, inconsistent, etc.
Immanuel Kant once addressed the practical value of keeping a supersensible category around (a la the ancient Greeks) purely as a refuge to project ideas with contended useful purposes upon; or when such was seemingly entailed by the very nature of a long-held popular concept, right, etc. However, this practical perspective did not include supersensible possibilities blatantly intruding upon the regularities of the empirical world (metaphorically akin to the geek designer of a computer game appearing before a crowd of the game's characters and performing undeniable miracles before them).
"But as will be shown, reason has, in respect of its practical employment, the right to postulate what in the field of mere speculation it can have no kind of right to assume without sufficient proof. For while all such assumptions do violence to [the principle of] completeness of speculation, that is a principle with which the practical interest is not at all concerned.
"In the practical sphere reason has rights of possession, of which it does not require to offer proof, and of which, in fact, it could not supply proof. The burden of proof accordingly rests upon the opponent. But since the latter knows just as little of the object under question, in trying to prove its non-existence, as does the former in maintaining its reality, it is evident that the former, who is asserting something as a practically necessary supposition, is at an advantage (melior est conditio possidentis). For he is at liberty to employ, as it were in self-defence, on behalf of his own good cause, the very same weapons that his opponent employs against that cause, that is, hypotheses. These are not intended to strengthen the proof of his position, but only to show that the opposing party has much too little understanding of the matter in dispute to allow of his flattering himself that he has the advantage in respect of speculative insight.
"Hypotheses are therefore, in the domain of pure reason, permissible only as weapons of war, and only for the purpose of defending a right, not in order to establish it. But the opposing party we must always look for in ourselves. For speculative reason in its transcendental employment is in itself dialectical; the objections which we have to fear lie in ourselves. We must seek them out, just as we would do in the case of claims that, while old, have never become superannuated, in order that by annulling them we may establish a permanent peace."--CPR
