I think that one of the most thematically significant purposes behind the title is the idea that the defense actually had a case.  It was quite a good one.  The entire exposition of the piece is how this case was about as close to certain as it could possibly be.  The apparent breadth and depth to eyewitness testimony against Adams makes it clear that the guilty verdict will be a foregone conclusion.  Greene's development of how the defense's case actually seeks to undermine the eyewitness testimony and exonerate the defendant is the crux of the plot.  The title's idea that the defense had a case and presented it well is also thematically significant because of the ending.  Even with the exoneration of the defendant Adams, we, like Mrs. Salmon, are left with more unknowing and more uncertainty.  The case for the defense provides legal exoneration and answers, but does little else for moral or ethical understanding.  When the bus hits one of the Adams' brothers and Mrs. Salmon sees the other one in agonizing tears, the case for the defense provides little in clarity.  It is here where Greene brings out a very postmodern reality that legal justice and moral or ethical notions of reconciliation can end up becoming two different entities.  It is here where the case for the defense becomes quite poignant and powerful.