Yeah, I had a similar thought with this.... keeps it in everyones mind that Wynne was in some way associated with a corruption/bribery trial (even if just as a witness).
I would imagination Brown et al would love for Wynne to go through with the suit. An opportunity for Brown's lawyers to ask some tough questions of Wynne on the stand (which would be political theatre to be sure), and then depending on the pre-trial discovery try and drudge up any dirt. Continued re-appearances in a court room regarding corruption in any capacity - founded or not- isn't a great look for a politician. 
Lasty, if Wynne backs down on the threat, then Brown jumps on it and frames it as an admission of guilt......