Blonde Justice and Western Justice (what is this “Justice” thing of which you speak?) have addressed the question of why a criminal-defense lawyer would not share exculpatory information before trial with the Government. WJ asked, “why would someone wait until trial to show the prosecutor exculpatory information like that?”
The Blonde’s answer (written before WJ’s question, but linked to in a comment) is that it’s a question of trust: if the Defense trusts the prosecutor not to try, in the time before trial, to wire around the exculpatory evidence, then the Defense might share the gem with the Government. If the prosecutor is one who does things like threaten witnesses with prosecution for not toeing the official line (mumble mumble FCLD mumble mumble), then the exculpatory evidence gets held back. And then, The Blonde writes:
… it comes out at the trial, in front of the jury — with absolutely no warning to the prosecutor. I’ve seen that happen a few times, and each time it was a very humbling experience for a very arrogant prosecutor.
But it is sort of fun to watch. They don’t see it coming and then BAM! it’s like a train wreck! And who doesn’t love a good BAM! moment in a trial?
The Blonde is talking about a tactical reason not to reveal an Nasty Little Surprise (NLS) to the Government before trial, even if that NLS is a bombshell that seems to demand a dismissal: because the untrustworthy prosecutor might woodshed her witnesses to defuse the bombshell.
Tactically, the criminal-defense lawyer has to decide whether, and when, to reveal his NLSes to maximum effect. Some NLSes might be bombshells that will change the outcome of a trial (like the “complainant” swearing that the crime didn’t happen), and some might be less earthshaking revelations that might affect the outcome of a trial. The former might, if revealed before trial, convince the trustworthy prosecutor to dismiss outright even what he believed to be a whale. The latter might shift the tone of negotiations and convince him to offer a more favorable plea agreement, or dismiss the case that he had doubts about before.
The Defense should (as The Blonde illustrates) not reveal either kind of NLS to an untrustworthy prosecutor.
A criminal-defense lawyer might choose not to reveal the second kind of NLS even to a trustworthy prosecutor if the parties are so far apart in their negotiating positions that a small shift won’t bring them to agreement.
All of that is tactics; strategy is another matter entirely.
There is a strategic reason (setting aside the fact that the Defense can’t be certain of the effect the bombshell NLS will have) for the Defense sometimes not to reveal the bombshell to the Government before trial: once bitten, twice shy. A defender who holds back a bombshell for trial one time contributes uncertainty to the Government’s case in every case from then on.
The less certain the Government is of its case, the better the Defense’s negotiating position. If the criminal-defense lawyer has a reputation for springing unpleasant surprises for the Government in the middle of trial, he’s going to get better plea offers than if he lays all his cards on the table before trial in every case.