I rarely post regarding fandom meta, but I am going to make an exception now, because I think this entire debate is based on a false premise: namely, that a warning is somehow functionally different from an advertisement.
To paraphrase G. K. Chesterton: a summary is only an advertisement rightly considered. A warning is a summary wrongly considered.
The truth is, anything that you reveal about a story outside of the actual content of the story serves as BOTH warning AND advertisement. When I announce that I am writing a Fullmetal Alchemist story, this warns off anyone who is looking for something different (e.g., one of my old Digimon fics), but it also serves as an enticement for those people who are actively seeking stories for said fandom. Similarly, I further "warn" people when I announce that my story is Scar/Ed (dude, this is a turn off to most FMA fans and don't I know it), but it also serves to attract that rare, rare subset of people who share my interests.
Me? I wouldn't have it any other way.
I know that there are plenty of people who are outraged that there are people who want to be warned for rape. But are those same people outraged when someone wants to be warned for, say, Star Trek? Probably not.
Even though this debate is often construed in terms of courtesy and kindness (and lack thereof), I don't really think this is the issue. Sure, there are the occasional dickwads who purposefully try to conceal the fact that they are writing shocking, triggering stuff. People who WANT to draw in readers pretty much for the sole purpose of fucking with their emotions and making them feel trauma. Those dickwads are being particularly vocal during this debate, presumably because it gives them pleasure seeing people getting worked up and upset over something that is "just an internet argument." These people are out-and-out trolls, and the best thing to do is just ignore them.
No, I think the problem is that for most people, writing summaries is difficult. And few people grasp that even when you are announcing shocking/disturbing themes, you are ADVERTISING. I think many people also feel uncomfortable admitting when they are writing to attract a certain audience: people who write rape are usually wanting to attract people who like rape as a kink, or who like rape as a source of drama and angst. It is very easy to feel self-conscious about writing rape, and therefore it is easy to displace this discomfort upon the people who are triggered by rape fic: to say that the problem is with those weakling fragile sissies who are such delicate flowers that they can't handle rape fic.
The fact of the matter is that writers who WANT to attract readers have responsibilities to properly advertise their fics. This has nothing to do with the sensibilities of the readers, at least not directly: not unless you want to blame readers for daring to have the audacity to have personal taste when it comes to their reading habits. The reason to mention "rape" in a summary, when rape figures predominantly in a fic, is morally NO DIFFERENT from the reason you mention what fandom you are writing for. Sure, this is information you can chose to leave off, and you're not committing a moral sin if you do so(presuming, of course, that you are not being deliberately misleading). But why set up an antagonistic relationship with your readers right off the bat? The reason you write a good and accurate summary is because you want people to read what you wrote. And you want to attract people to your writing. Warning off people who are NOT interested in your sorts of stories is a SECONDARY side effect, one which acknowledges that no writer is capable of pleasing the majority of people all of the time.
I should mention that although I said earlier that fic summaries are not about courtesy and kindness, this was partially untrue. A kind and courteous person will naturally warn for the appropriate things, and when they somehow don't, it is usually because of an oversight and can very easily be fixed once the problem is brought to their attention. The flip side of this is that an unkind and rude person will be aggressive when confronted on the issue of warnings, and take it as a personal affront even when it is not intended to be the case. Not every suggested warning that a reader asks for retrospectively needs to be honored, but if a reader bothers to take the time to wonder why a warning was not given, I think that any writer who actually cares about her readers will take the time to explain (kindly!) the rationale for why such-and-such warning was not given, and (if necessary) why it will not be given.
A summary is by its very nature a short, limited thing. By definition it cannot include mention of all possible "triggers" (or, put another way, "enticements"). But if a writer refuses to provide a useful summary for a fic and is stubborn about it, said writer should not be surprised when readers provide a summary FOR them (in terms of either reccs or flames), or when readers avoid the fic altogether. No reader should have to justify their triggers, kinks, preferences. Nor do their triggers, kinks, preferences etc have ANYTHING to do with their strength or value as human beings. It would be helpful if more writers didn't treat the relationship with readers as an antagonistic one and to simply realize that a lot of what accounts for differences in opinion are merely matters of taste.
We, the writers, need not feel guilty or feel like a bad people for writing almost any kind of triggering fic. As long as our writing is not intended to stir hatred and criminal activity, it is not an evil act. But we, the writers, also need to acknowledge that the idiosyncratic preferences and squicks of various readers DO NOT have any bearing on those readers in terms of their character. We need to stop treating crime victims who shy away from triggering fics as weaklings who simply "can't handle it." Dude, if they don't want to read those stories, it's not weak for them to simply say no. Just the same as it is not "weak" for someone who might look at my Scar/Ed fanfic, think "ew, gross," and pass. IT'S ALL ABOUT TASTE AND THAT'S ALL IT IS. Give readers a break already.
If we, the writers, take pains to give the readers a break, we'll most likely find that they are more then willing to return the favor. We are the ones doing the courting; it is our responsibility to take the first step, and court properly. To me, this is simply common sense.
(post to remain public)