Sandusky wasn't going to win at the prelim. Sure, he gave up the right to get the accusers' versions on the record and under oath and cross-examine them, potentially exposing weaknesses that could result in a more favorable offer from the proseuction and provide him with impeachment fodder. But the accusers have already told their stories to investigators (who put their statements into written reports) and in many instances, to the grand jury. Impeachment at trial is still viable. Also, by not testifying now, the prosecution may be deprived of the chance to evaluate the accusers' weaknesses as witnesses and rehabilitate them before trial. And it avoids having the witnesses become acclimated to testifying and more sure of themselves on the witness stand. (Studies have shown that juries tend to believe confident witnesses.)
Defense attorneys frequently waive preliminary hearings (mostly because so many prosecutors make offers contingent on waiving the hearing. Even if the defense isn't ready to take the deal, the prosecutor may require the waiver just to keep an offer or negotiations open.)
Also, the prosecutor agreed not to try and increase Sandusky's bail if he waived the prelim. That's important for a few reasons. More victims may come forward and more charges may be brought, which otherwise could result in prosecutors seeking to increase or deny bail. Also, reviewing evidence and preparing a defense is always tougher with a jailed client. And with Sandusky looking at a potential life sentence if convicted, staying out on bail is critical to him.
The media and commentators may be disappointed, but it was probably the best move for Sandusky.