I find the reference-question tag useful. Here's how it is useful: it is useful for me to identify resources that I can point new users to. I realize it may not be super-useful to new users, since they probably won't know about its existence.
Here's the benefit of reference-question, from my perspective.
We've identified a handful of topics that often seem to get asked (how do I prove that a language is/isn't regular, how do I compare functions using big-O notation, etc.).
Folks have written one very good question and answer on that topic: a reference question.
Then, whenever anyone asks about that topic, they can be pointed to that reference question; if their new question is already answered by the reference question, we can close their question as a dup of the reference question.
So far, so good. No special tag needed. But here's where I find the tag useful. I don't have a list of all reference questions in my head. Often, I recall that there's an excellent reference question on the topic, and I want to search for it so I can point someone to it (or close a question as a dup of it), but finding it is a pain. Because we have the tag, I can just search on that tag, and then I will quickly find the reference question.
So, yes, the tag is useful, and we should not nuke the tag. The meaning of reference-question is basically: this is a FAQ (a frequently-asked-topic); here's a question that we think gives an excellent overview.
If the tag exists, we should be a bit selective about how the tag reference-question is used. We should only use the tag where we think it fits the criteria above.
The BFS vs DFS question is, in my opinion, not a good example of something that the reference-question tag should be used for. There is no evidence that this is a frequently asked question, no evidence that we'd want to use the BFS vs DFS question as a frequent dup target, and it doesn't provide a good overview that will answer many questions all in one go. It's just totally different from the other actual reference questions.
That said, if others want to get rid of the reference-question tag, I'm fine with that. I'd prefer to keep it because I find it handy and I use it frequently, but I won't stand in the way of removing it. But in any case, let's not pollute it by adding the tag to questions that don't fit the criteria.