I disagree. Visual shorthand is all well and good, but I'm more inclined to see laziness of design. Just as motion pictures are kinda supposed to be an artform, there's really no law prohibiting a movie's promotional materials (including the one-sheets) from being art as well. I suppose the people who actually design the posters consider themselves some form of artist. Probably says "Art Department" outside their door.
One could extend your argument in defense of these posters to the movies themselves, and say that there's nothing wrong with making a movie that can be completely defined in terms of the successful earlier films which it shamelessly rips off, that such homages are simply using comfortably familiar narratives, situations, stock characters, and derivative dialogue so the audience can more easily digest the product without having to think very much (or at all) about it. And though that may be perfectly true, few would admit to actively seeking out regurgitated crap like that.
The fact is, the production of a feature-length motion picture (especially a studio one) is a horrifically expensive risk, and so the studios tend to mitigate that risk as much as possible by greenlighting what they feel to be relatively safe and familiar bets. In the same fashion, the advertising, publicity, and promo departments are inclined to want to communicate the movie's charms as quickly and efficiently as possible, as you say. But really, they don't need to keep using these same tired, familiar old tropes. It wouldn't kill them to retire the ol' "frame the poster between a standing woman's spread legs" thing, any more than it would kill Hollywood to retire the creaky old plot chestnut where the remote self-destruct doesn't work, so somebody's gotta crawl down into some remote tight-fitting closet to pull the manual handle.
It's fine (and funny) when the poster for Puss In Boots recalls the poster for Unforgiven; it's supposed to, and it's a legitimate joke. Same thing with the poster for Texas Chainsaw Massacre 2 aping the poster for The Breakfast Club. That right there is comedy. But I think too many of these movie posters resort to cheap, derivative, lazily referential imagery. Boing Boing has more than once featured posts about Polish movie posters (many for familiar American movies) that illustrate just how original and arresting an artform it can be, when it doesn't stoop to instant-eye-catching lowest-common-denominator dreck. I recommend using BB's search function to check them out, since I think I've run out of links.