Having read all of your replies so far, I first want to say that in no way does distinguishing what is canon and what is not canon limit what you can discuss regarding lore, or the possibilities of those discussions being relevant to what has yet to be revealed in lore so far. I am not trying to end your fun or limit what you can discuss about lore. Lore is meant to be balanced, in that the games offer different perspectives on the truth. As such, no lore is inherently right or wrong; it's all up to interpretation.
However, there is a dividing line between what is canon and not canon. Take for example, M'aiq the Liar in Morrowind's dialogue about weresharks. Though M'aiq is not the most reliable source, and we haven't seen any weresharks in the games (that we know of), weresharks are definitely mentioned in lore, and so, are canon. Does this mean weresharks are real? Maybe, maybe not. Like all lore, it's left to interpretation. The possibility of weresharks is no more or less likely than the possibility of mammoths being female giants, but the possibility of weresharks has been referenced; the possibility of mammoths being female giants has not. One is canon, one is not.
The same thing cannot be said about unofficial lore. Unofficial lore can only reference what is already established, not the other way around; you can't make up a theory and claim established lore is referencing it. Developer texts are a different matter. The established lore may truly be referencing their current texts. But because of the uncertainty of future events, the developers cannot say, "All of my lore texts posted in these boards will definitely be referenced in future Elder Scrolls games." If they could, there'd be no problem.
Defining canon may seem trivial to you, but it is important for the same reason many of you have deemed it unimportant; it creates true equality within discussions about lore. This does not mean that all lore is canon, but it does mean that all lore is equal in worth. That is, someone's lore cannot be dismissed simply because that person is not a developer. Likewise, developer texts cannot be dismissed simply because they are unofficial. All lore is equal in its possible validity, but not all lore is canon.
It is a definition, not a restriction. It is because of the confusion of what canon is, as apparent in your mixed replies, that defining it is important. Without a standard to agree upon, in any discussion of lore you have no foundation for your argument. There is no mutual ground to begin a proper debate without first defining the terms. You risk misunderstanding one another because you do not share the same basic definition of lore. Without the same values, someone could say that Kirkbride's texts are not worthy of discussion because they are unofficial, which would be unfair. But if you both agree to an objective standard - Bethesda - and by doing so agree to a definite set of canon lore, you are free to discuss all lore as equal in possibility.
You may be asking why all of this is necessary. Well, it is because even though many of you may already view all lore as equal, you each have different opinions of what lore is canon, and in some cases no opinion at all. You may see unofficial and official lore without any clear perspective. It would be like two people discussing how far east Alexander the Great trevelled without first agreeing that he was a real person and not a historical myth. Would both take the discussion seriously? And if one person was just humoring the other, what does that say about the discussion itself? Was it a waste of time? How then can we discuss lore without defining what it is? How do we separate a theory based on evidence from pure speculation? Reason is a double-edged sword. Having clear definitions makes starting a discussion easier and prevents misunderstanding.