If someone kills someone else, I'd expect to see a trial in most cases. The only reason not to would be overwhelming, clear evidence of innocence.
Aren't the different witness statements by so many folks enough in this case to go to trial? There's no video of what happened.
I guess it could be done like so many rape cases, where there's just not enough evidence to convict, even if suspicions are that the guy is guilty. That makes me sad that so many rapists don't even get charged, though. I like to see the victims have a day in court (if that's what they want). Ferguson may just want its day in court. That seems reasonable to me.
I like the 'grand juries will indict a ham sandwich' bit. Trials are very useful to understand what happened, for transparency, imho. That the case looks like it would probably lose is less important to me, although costs can make it become more important in reality. If there are victims who feel that justice is only served through a trial, I get that. If nobody's much bothered by what happened and the case looks shaky, then that's different.