Yea, but in this case there's an additional reason. If Z testifies among the train wreck of the explanations he's already given, the jury will see first hand how peculiar his character is.
"Train wreck" That's good. The only train wreck here is the prosecution's case. Zimmerman has been pretty consistent.
Liars always are.
So Zimmerman's explanations are a train wreck because they've been consistent all along? Are you on drugs?