The genres in which the future imperative predominates --- proverbs, statements of law, recipes (supposedly: I've never seen one) --- have in common that they state precepts which are thought to hold generally and which are intended to be followed always: Missourians are not to believe that the people's safety is to be the supreme law today but not tomorrow. I would say there is good reason for thinking that it is the semantic difference between a folded-arm precept and a pointed-finger order which dictates the form in these genres.
If the verbs which regularly take the future imperative variously meant, say, "to itch", "to eat", and "to sprout", I might agree with you that it is a morphological happenstance rather than a matter of semantics, but, since it can't reasonably be considered coincidental that meminī
, and (if Gildersleeve is right) habeō
have such similar meanings, it only makes sense to seek the reason for their shared form in their shared meaning. Admittedly, the reason which I stated for their taking the future imperative was my own speculation and probably wrong, even if I find it satisfying and mnemonically useful. And I certainly don't think that semantics could be the whole story here, as poets are want to choose words and forms partly for their metrical shape and their musical quality, and all careful authors can be seduced away from the pedant's correctness by the peculiar ring of a word.
I think you must have linked to the wrong google search there, as it shows results for the query "Horace", whose existence I don't need convincing of! Anyway, the Packard Humanities Institute's site
, with its near-exhaustive collection of classical Latin texts and rather lovely concordance feature, is the best venue for such searches now. Here are 117 instances of habē and habēte
versus 128 of habētō and habētōte
. A search for scī and scīte
returns mostly false positives, but on just a quick scan I did spot at least one instance of a present imperative of sciō
: Ovid, Met.
, XV.143, mandere vos vestros scite et sentite colonos