I almost always use "Quote" when replying to a post. It lets people know which post I'm replying to and,  
because I frequently trim the quote down, it focuses attention on the specific point or points I'm addressing.  
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I also prefer to just use the same subject line as in the post I'm replying to. But there is a strange behavior.
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When I click "Quote" to reply to a post, SOMETIMES the subject line is copied into MY subject line. Other times, my subject line remains BLANK. Does anyone know what triggers one behavior or the other?

I don’t think it matters whether you click on reply or quote, but if you reply to an OP you get a blank subject line. If you reply to a reply to an OP, the subject line is pre-filled with the previous poster’s subject.