If you were the hostess, and every guest reminded you that they're vegan/eat only free-range/are on Atkins/are allergic/are diabetic/are celiac/are kosher, etc., etc., are you going to prepare a separate dish for each and every one?
I would and I have. The most effort I have gone to for a party was one where I had invited
- one friend who had celiac disease
- one friend who keeps kosher
- three friends who are vegan
- one friend who is allergic to all members of the nightshade family (eggplant, tomato, potato) and peppers
- one friend who is allergic to all tree nuts
Obviously each hostess is going to have a different level of willingness to go out of her way for her guests, and guests should exercise some common sense in offering information to the hostess about what they cannot or will not eat. If it's an intimate dinner party with only two couples, the guest should definitely mention restrictions. If it's a big buffet with twenty guests, they might only want to mention restrictions once the party starts, in the context of "Does this dish have an ingredient that will make me sick?"