I have always been mystified at the unqualified "open to discussion or debate" meaning of moot, and consider it to be an error (at least if used within the confines of American English). I've never seen it used that way, without the accompanying notion that though the question itself is unresolved, it is not useful to debate in the current context because of lack of relevance. If someone declared that a point was moot, then followed up with "... so let's debate it right now!", I would either seek help for the individual or avoid further contact.
And that's the way it is used in U.S. jurisprudence, at least-- cases are often dismissed for mootness, and a moot point may be defined as an "issue presenting no real controversy", "a subject for academic argument", or "an abstract question that does not arise from existing facts or rights" (example definitions pulled from The Free Online Dictionary). There is accordingly a fairly rich body of case law supporting the legal doctrine of mootness, and outlining its limits and exceptions. Practically speaking, the issue of mootness is not whether the issue is truly just abstract or academic, it's whether the court thinks it can finagle its way out of addressing it in the context of the facts of a specific case. For example, even though a case is initially considered moot, it may be addressed by a court if it is capable of repetition, yet evading review, i.e. though in the case being considered the issue no longer exists (usually because the real-world situation has resolved by the time the case is reviewed), that will keep happening if the court doesn't agree to resolve the legal issue.