IANAL but the rules of evidence are pretty strict. It isn't enough to just state vaguely that "everybody knows why Trayvon was in Sanford." The SA will use the family members to establish why Trayvon was where he was, that he belonged there, that he went to the 7/11 (when and why), etc. All these elements have to be established, even though they are all known to us.
I expect the SA will also use TM's mother, at least, to testify that the screams came from her son. To some jurors, a mother's word will be more convincing than any expert.
The family may also be used to testify as to TM's character, if the SA wants to show it is unlikely that TM "jumped" GZ in the dark.
In a different vein, GZ's friends may be used to demonstrate that GZ has told conflicting stories about the incident, thus indicting GZ's credibility. I think such stories would be exceptions to the hearsay rule.