I believe the exact purpose for short adjectives is for the use as predicates, in contrast to a common adjective, which affects a noun that will go on to function with a verb(not быть). For example, "Книга старая... (verb is assumed to follow)" vs. "Книга стара."( - would likely be used I'd think) As far as I understand the sentence functions standing-alone, because the finite, as opposed to infinite/infinitive, verb has already come up: the omitted form of быть which is assumed to come with short adjectives. Ultimately adjectives made from verbs, passive or active, are full-fledged adjectives on the grammatical level in every way, though colloquially certain uses may be completely strange. When I first learned of verbal adjectives in Russian, I had the same question. Could one use the present active as an equivalent for English present progressive, e.g. "Птица летяща(я)." to mean "The bird is flying.". No. Essentially it makes no sense. I think the topic relates a lot to the English loss of the "passival" voice. In the olden days, one could say "the house is building" in order to avoid use of a real agent/subject. This was the passival. The structure "The house is being built." (the now standard passive voice replacement for the now unheard-of passival) seemed confounding in every way to many people, as they would understand it meaning: "The house is, being built, ...............(quite larger than the schematics suggested)"(Also, for comparison, the modernday, very different interpretation of the same words "The house is being built quite larger than the schematics suggested." which means someone is building wrong right now, as opposed to built the house wrong already). "Being" was supposed to only be an active adverbial participle, not a verbal adjective that shares the same form. The same expectation of the sentence not being over because of the clause structure and implications made by verb forms can be seen there. I actually don't know what I'm talking about as far as the use of short adjectives, and probably haven't answered your question.