The other thing is, when you're serving a limited menu, it seems more reasonable to me to be as specific as you can about what's being offered. There are many reasons why the HC might choose "three pre-plated entrees" over a buffet, and there's nothing wrong with that, but by its very nature it's a more limited selection than a buffet and thus more likely to run up against people's dietary restrictions and even preferences.
Even if you had the exact same food on the buffet as would have been plated, and only that food, people would have more freedom to pick and choose both the food and the portions--they could get themselves a larger portion of scalloped potatoes which seem safe, skip over the green beans once they see there's bacon in them, take half a portion of chicken because they're not sure about it, and fill the rest of their plate with green salad. Whereas with a pre-plated meal they're limited to the entree and sides already on their plate, unless there's someone they feel comfortable swapping/sharing with.
And I certainly think adults are ultimately responsible for their own food, and even if they walk into a situation blind they should be able to deal with it somehow, even if it's by sneaking off to the bathroom to eat a granola bar they brought in their pocket. But if the HC/hosts know more specifically what the pre-plated (or even buffet) food will be, and what other "off the menu" options are available, I don't see any reason to keep that information secret from the guests. Definitely put it on the response cards if it's known by then, or on the wedding website or something.
The other thing I think about is the expectations for the event. If I knew I was going to an event where food was not provided, even if it occurred when I usually ate, it would be no big deal to handle that myself by eating before/after, discreetly snacking in the middle, etc.. But when you open the door and say "food is provided" people start to think it ought to be food they can eat--which is not unreasonable, it'd be horrible if the only "food" was stale chips and expired dip, for example. That would not be proper hosting. But from there it's a slippery slope--red meat, white meat, seafood, no meat, no animal products, gluten-free, nut-free, low-fat, kosher... I think it's just hard for a host to determine how far they need to go to accommodate dietary restrictions, and for a guest to determine when it's polite to mention their restrictions and when it isn't. No reasonable host wants to seem unwelcoming, and no reasonable guest wants to seem demanding.