With bombing admission, why is Tsarnaev on trial at all?

BOSTON (AP) Boston Marathon bomber Dzhokhar Tsarnaev's lawyer startled a packed courtroom during opening statements at his federal death penalty trial by bluntly admitting that he committed the 2013 attack with his brother, raising some thorny questions, mainly: Why have a trial at all?
Some legal experts and a bombing survivor weigh in on the strategies at work and the reasons why the trial is moving forward:
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Q: Why didn't Tsarnaev's lawyers persuade him to change his plea to guilty if the defense acknowledges he did it?
A: ''A plea of guilty would result in them waiving all their appellate rights. There are some issues - the venue issue, the makeup of the jury - that they might want to bring to a higher court, so that's another reason they probably wouldn't plead guilty,'' said Boston College law professor Robert Bloom. ''Further, they recognize that this is a two-phase trial and they want to do what they can during the first phase so as to start to make their arguments for the second phase ... the penalty phase.''
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Q: Why put the victims through this?
A: Rebekah Gregory, a woman who lost a leg in the bombing, was one of the first survivors to testify during Tsarnaev's trial. Hours after her testimony, she posted a letter to Tsarnaev on Facebook, saying facing him in court actually helped her.
''TODAY ... I looked at you right in the face ... and realized I wasn't afraid anymore. And today I realized that sitting across from you was somehow the crazy kind of step forward that I needed all along,'' she wrote.
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Q: Why not just have a penalty phase for the jury to decide punishment instead of going through both the guilt and penalty phases of the trial?
A: ''A competent death penalty defense lawyer - during the first phase - will present themes that are relevant to guilt - like diminished capacity, diminished responsibility - that are also entirely consistent with what's going to be said during the mitigation (penalty) phase,'' said Eric M. Freedman, a death penalty specialist and professor of constitutional law at Hofstra Law School.
''Death Penalty Defense 101 is to present a unified theme through the guilt phase and the penalty phase. ... Therefore, you are ill-advised to argue in the first phase, `My client is innocent,' and in the second phase, `My client is very sorry for what he did.' That's completely ordinary and being carried out in textbook fashion here. They are doing precisely what they are supposed to do.''