It all depends on the subsection - take Travel for instance.
If someone wants to know how to get from Gare du Nord to Gare Montparnasse, and Buenchico comes along with one of his very helpful and detailed answers I see no reason why that shouldn't be awarded BA even though it's the only one.
In other topics where debate plays more of a part - News and R&S for example it would only be fair to award BA when the thread has run a reasonable course.
For Crosswords and Q&P there is usually just one correct answer, and sometimes as many as 4/5 of these can appear in the first minute or two, so it's up to the poster really - BA is only awarded in a relatively small percentage of questions in these subsections.