I think you can make the self-defense case without it. The neighbor testified that Martin was on top of Zimmerman doing the "ground and pound" (funny how MMA has entered the public lexicon...).
And if I'm on the ground getting hammered, and there's no indication that the dude's going to stop, I think it would be pretty reasonable for me to believe I'm at risk of serious physical injury or death.
Here is something that has bothered me ever since I heard Zimmerman's side of the story. He claims that, and the testimony of the eyewitness today (the one who knew who was on top and who was on bottom even though he can't conclusively say if punches were being thrown because it was too dark) Martin was straddling Zimmerman raining punches down on him. Eventually, according to Zimmerman, the punches stopped and Martin began bashing his head into the concrete sidewalk. While all of this was happening, Zimmerman contends that Martin was trying to go after the gun that was holstered on Zimmerman's hip.
Here's the part the bugs me ... If Martin's right leg is over one side of Zimmerman's body and his left leg was over the other side, we can all agree that Martin was in a dominant position. I can envision Martin going after the gun, although not at the same time he was relentlessly pointing on Zimmerman's face. But that isn't even the thing that bothered me. It was that the guy in the dominant position, with his legs blocking a clear path to the weapon strapped in a holster to Zimmerman's side would have had a much easier time getting the weapon than Zimmerman, no? If Zimmerman pulled the weapon only after getting beaten, it would mean that he had to reach around the leg of a guy who was beating the crap out of him, then back toward his body, having the incredible dexterity in his fingers to not only reach the gun, but to pull it back toward the butt of Martin to pull it out of the holster, position the gun in his hand so that it could shoot, pull it back around Martin's leg and pull the trigger, all while Martin was supposed to be going for the gun himself from the dominant position in the fight and/or pounding Martin's head into the concrete. Either Zimmerman had the hand dexterity of a magician and the flexibility of a contortionist, or this story does not seem plausible.
This coupled with the other incredible inconsistency I spoke about earlier, when he said he didn't think he hit him when he shot and when Martin fell backwards (presumably getting off his chest to do so because he was so startled by the gunshot from the weapon he already knew existed because he was "going for it") and Zimmerman jumped on top of his chest (several witnesses saw him in this position) and held his hands to the ground. However, when police arrived Martin was face down in the grass. Did he flip him back over and forget to tell anyone?
Zimmerman's story is consistent with a guy who killed someone and came up with the most plausible story he could on the fly (perhaps while he was pacing back and forth at the crime scene (as witnessed by the homeowner in the adjacent unit) with his hands on his head. Zimmerman's story is really, really hard to believe.
This is to say nothing about the fact that there wouldn't have been a confrontation in the first place if Zimmerman didn't follow Martin (ignoring the dispatcher).