"Think it might turn out to be a precedent setting case?"

Probably not.

When dealing with legal news, you really shouldn't put much stock in your regular media - they routinely make mountains out of molehills, misunderstand proceedings and make other goofs. Unfortunately, complex legal matters don't boil down into a 3 second soundbite very well and the result seldom bears much resemblance to reality. For example, in nearly all high-profile cases the media will announce as breaking news that fact that a defendant pled "not guilty" at their first appearance in front of a judge. This is not news - it happens in every criminal case. If such a defendant wanted to plead guilty at their first appearance, that would be news. The simple fact is that very little is usually known about the case at arraignment and any attorney who would allow their client to enter anything but a not-guilty plea would be probably be committing malpractice.

As for the reporter's statement "wants his client to be granted leniency because she is the descendant of slaves:"

First, the statement appears to be a reporter's interpretation of information lifted out of more than eleven pretrial motions. We know nothing about the context of the information upon which she bases the quote. In fact, the only words in quotes are the phrases "descendant of slaves" and "experienced overt and cruel discrimination in her state on the basis of race." The rest is all paraphrased. We don't know if those phrases came from the same sentence, much less the same motion.

Second, it sounds as though most of these are pretty standard motions. It is very common in death penalty cases to file motions attacking the constitutionality of the procedures, jury instructions, jury pool, orders, etc... They usually are denied out of hand, but preserve the record for appeal in the event that the issue does eventually go your way in a higher court. Examples of this is are the recent U.S. Supreme Court decisions regarding executing the mentally retarded, juveniles and judges imposing the death penalty when the jury could not reach a verdict. The quotes makes sense if they are used in the context of tracing the historical development of a current practice that the defense is attacking as violating one or more provisions of the Constitution. For example, they may be arguing that the indictment should be dismissed because the grand jury that handed it down was not representative of the community because it excludes a higher proportion of African-Americans due to systemic racism. As a stand-alone proposition, I seriously doubt the "my g-g-g-grandma was slave" argument is going to cause some sweeping change in Death Penalty jurisprudence.

Third, if this really is an argument intended for the jury, it is doomed. If they argue it in the guilt phase, they are conceding that Hughes is guilty. They might try to argue it as a mitigating circumstance in penalty phase, but I doubt it will get much traction with a jury anywhere, much less Mississippi. The most logical use at trial would be as some sort of insanity or diminished capacity defense, but those are rarely successful.