Providing context is, as a rule of thumb, a bad idea. It takes time to read it, and that's the exact opposite of the point of a one-liner. If it's necessary for answering the question, then chances are the question is too broad or poorly targeted: your recipient ought to already have the necessary context. That's what makes them useful for asking your question.
That said, it's just a guideline, which means break it when you feel like breaking it.
Everyone is going to have a different threshold for what seems rude, what seems worth answering, and so on; you kinda have to go with hit or miss. Only this past week, I actually got a response back from an author I emailed over a month ago. I had stopped expecting a response two days after I sent it off and moved on, so it was a very pleasant surprise.
My last email went something like this
Subject: One line response needed only Body: Hi X,
Could you please tell me what you consider a healthy user activation rate?
Thank you
Regards
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That's it. Of course, that's not how I would email everyone.