Very true. I've always been under the assumption that DKR is not canon. Just because it's a crowning achievement in the super hero doesn't necessarily mean that it's canon. Writers just like making references to it because it's an alternate ending for the Batman mythology, just like how Magog from Kingdom Come appeared in the Superman series.
It doesn't matter if it's canon.
It's the most influential comic of the 80's. It pretty much defines who Batman is from that point on.
It's canonical status is a moot point.
I personally think that Batman Year One is more influential to shaping Batman. I wouldn't go as far as saying that DKR is the influential comic of the 80s (but definitely the second)...that title is sole property of Marvel's Count Duckula.
Seriously, my vote would be Watchmen. It is a much better story, and this is coming from someone who swears by Frank Miller.