You wouldn't be talking about me, would you? There are a few responses I'd have to your answer.
Firstly, like Stack Overflow, there's nothing wrong with building up a repository of information on this site. Whilst we ideally don't want completely obvious plot questions (and yes, there are a fair few of them), there are many people who genuinely don't understand plot details which have multiple interpretations and so ask them here.
In the case of the former, i.e. this question about The Graduate, it will help redirect future traffic to the site (especially good since it's still in Beta). In the case of the latter, anyone with a genuine question about a plot explanation deserves an explanation, e.g. these Groundhog Day and Mulholland Drive questions.
As to references to IMDB, Wikipedia etc, I understand your concerns, but I think it's a valid way of answering questions. On a site like Stack Overflow, answers that are provided are testable and often definitive. If I want to know if an answer is correct, I can simply copy it into my program, run it and find out. If I ask a question about a specific feature, I'll often have the documentation of whatever programming language I'm asking about quoted back to me.
Relating to that, on this site a link to Wikipedia is perfectly acceptable in my view if it is simply to give a quick recap of a part of a story. A link to the script is perfectly acceptable to demonstrate a quotation if the OP has taken it out of context/misunderstood it. A link to other reputable sources, e.g. academic articles, twitter feeds of people involved with movie, interviews, etc must be considered acceptable as they back up the argument being offered in an answer and help make it definitive.
From my point of view - when I answer questions here, I always want them to be definitive. There are so many sites out there that people ask movie questions on and the majority of the time the answers they get back are highly subjective. Whilst an element of subjectivity is present in almost all answers, it's nice to be able to point to a director interview, or a line from the movie, to almost entirely eradicate any doubt as to the purpose of a scene or plot device.
Finally, straight from the FAQs:
Movies & TV Stack Exchange is for Movie & TV enthusiasts and experts
Anyone with a casual or hardcore interest in Movies & TV is welcome and the site exists to provide a question and answer platform for anyone with an interest in movies and TV who have questions which fall within the scope of the site as defined in that link.
Meant to bring this up yesterday, as it was bugging me, but I forgot. Whether you agree with linking to answers or not, the word plagiarism is completely inaccurate to describe it. The fact you are linking to a source is the exact opposite of plagiarism.