I just stumbled across this question and thought I'd clear up what happened! No, you were absolutely within your rights to use the Lethifold point in your answer and there was no need to credit me at all. I don't mind either way but I was actually a bit surprised when you mentioned me in your answer. If anything, I was copying you, not the other way around!
The only reason I mentioned the Lethifold was because I was looking at other questions to do with Patronuses and ran across this comment - from yourself! - dating from 2012. This was, as far as I can tell, the original mention of the Lethifold reference on this site. My viewpoint in the comments-discussion was influenced by this question by David, also from 2012. So you were (unintentionally, it seems) actually citing yourself!
I only mentioned the Lethifold for completeness's sake because it essentially put a hole in my argument that a Patronus can only interact physically with a Dementor and nothing else. I had to look up what a Lethifold actually was on the Harry Potter wiki and would be quite happy to wager that your Lethifold knowledge outstrips my own.
My reference in the comments to the Lethifold was, as @Richard says, really very fleeting and circumspect anyway so I don't think that I'd be justified in complaining even if my point was original and I did feel hard done by (in reality, neither is the case).
As for the main question, I agree with the principle that if someone feels strongly about a point-of-view then they'll answer as a question rather than a comment anyway. Someone taking up a comment as part of more fleshed-out answer is perfectly entitled to do. Credit can be given at the answerer's discretion but in my view is not strictly necessary.