I understand that, but she would not be in jail now, and that would leave the burden on the prosecutor to be able produce other evidence to indict her. the media would just bury the "she took the 5th, it must be her" story (it would have to come from a grand jury leak) as they do with all the rest - they would just demonize Fitzergerald as a "rogue" prosecutor, etc. we all know the drill.
She gets more media exposure this way, and she'll walk out of jail in a few months as a "martyr" to their cause. Think of this as an investment on her part -- four months in jail in exchange for a lucrative book deal and a tour of the talk-show circuit in October.