"Is X useful?" is a natural question when trying to decide if one should use/undertake X. Sadly, it's not a very meaningful question when it comes to doing actual research.
If you take a question like "Is X useful (for Y)?", in most cases, if you break it down, the question is asking "Has X ever been useful to at least one person in history to accomplish Y?" Clearly that's not a very interesting question. The fact that someone, once, found it useful to recite grammar rules blindfolded while swinging from a trapeze may technically make this learning technique "useful" in the strictest sense. But what we want is more broad use-cases and patterns.
In a field as nuanced as language learning, X is likely to be useful in some ways, for some things, for some people, some of the time, and unuseful, even detrimental, for other ways, for other things, for some people, at other times.
When you look at the motivations that drive questions like "Is X useful?" what's usually going on is that a person is trying to determine "Is X useful for me, in my situation, at this stage in my learning?" So one approach might be to require all of these details in every such question. I think this would make each question "too localized" to be of broad value, and would be very tedious to boot.
I offer a couple of possible approaches, which might be selected depending on the OP's preference:
1. How is X useful?
This encourages answers which are relevant to a broad number of future visitors, while still answering the OP's question. It no longer hinges on specifics of the OP's situation, but a good answer will still apply to their situation.
Similar variations could include "What are the pros of X?", and "What evidence supports X?" And the negative approach is also valid: "How is X detrimental?", "What are the cons of X?" and "What evidence contradicts X?"
2. Is X useful for Y?
In some cases (perhaps a minority), Y may be so well defined that it makes sense to ask this sort of question. But we need to be careful with Y. A Y of "learn a language" isn't specific enough. If Y is "improve vocabulary recall times", it may well be specific enough. If the question is too broad because Y is not well defined, or is not specific, I'd suggest switching to format #1 above.