According to NOLO.com the reasons why a defendant might waive a preliminary hearing:
[FONT=&]A defendant may decide, after consulting with counsel, to waive the prelim. This allows the case to proceed to trial (though not immediately). The reasons the defense might waive the right to a preliminary hearing include:[/FONT]
[FONT=&]The defendant intends to plead guilty and wants to avoid publicity (and expense, if the defendant is represented by private counsel).[/FONT]
[FONT=&]The defendant is guilty of more than the charged offenses and fears further charges from the potentially damning evidence that may come out at the preliminary hearing. Also, if the facts of the case are particularly nasty, and the defendant plans to plead guilty anyway, the less the sentencing judge hears about the facts, the better for the defendant.[/FONT]
[FONT=&]The prosecutions case is strong, and the defense fears that prosecution witnesses may become so entrenched in their positions once they testify under oath at a prelim that they may become angry (or angrier) with the defendant and possibly refuse later interviews requested by the defense as it prepares for trial.[/FONT]
[FONT=&]The prosecution intends to call witnesses at the prelim who may be unavailable at the time of trial. If the prelim goes forward, this testimony will be available in the form of transcripts from the prelim for the prosecution to use at trial. By waiving the prelim, the defendant may prevent the testimony from coming in when trial time rolls around.[/FONT]
[FONT=&]The defendant wants to stall in the hopes that by the time the case comes to trial, the prosecutions witnesses will have either disappeared, forgotten, or become confused about what happened during the alleged crime. In this situation, the defense may waive the prelim and move for several continuances (delays) of the trial date.[/FONT]