Prior inconsistent statements play a critical role in undermining the credibility of a witness. More often than not lawyers would want to use prior inconsistent statements to show that the witness should not be believed. However, if the original statement is more damaging than what was said at the preliminary hearing, a lawyer may tentatively decide not to raise the initial statements to the police and go with the new and improved preliminary hearing version. I say tentatively because it is possible that by the time trial comes around there may be yet a third version or a sudden recollection of the original version. If the witness is unable to repeat the critical points of the story with any degree of consistency than you have a strong argument that it's all lies and the lawyer may be more willing to raise the original (though incriminating) version to prove that the witness is a flat out liar. If the first (police report) and third (trial) versions are very similar, whereas the second (preliminary hearing) version is different on some critical point, your lawyer may decide to emphasis the inconsistencies between the preliminary hearing and the trial without ever mentioning the police report version. By the way, it would be improper for the prosecutor to try and get in the comments made to the police – it’s hearsay and amounts to bolstering the witness.
Also, you should keep in mind that not all inconsistencies are important to the defense. A lawyer can lose credibility with the jury by focusing on minor discrepancies; it makes the defense looking petty and (worst yet) desperate.
Whatever your lawyer's reasons for minimizing the significance of the original version, you are entitled to an explaination. If the lawyer's strategy is sound s/he should be able to help you understand. I would recommend that you sit down with your lawyer and ask for that explanation.
If you have lost confidence in your lawyer you need to fire him, or, if he is appointed, ask the Court to relieve him. That said, the lawyer knows more about criminal procedure than you do and your question reveals no basis for your challenging his decisions. You should work with him, not undermine him, and accept that if your were a criminal defense genius you won't be the defendant but the lawyer.
While the police reports themselves are not admissible they are often very good areas to find inconsistent statements. Therefore I would agree with you that this is an area that should be used as it may be helpful to your defense. Basically the line of questioning is that these statements were made at or very near after the time the incident happened, therefore in theory those statements should be viewed as most reliable.
Why there is a change in testimony is an issue, but at the very least these differences should be brought out. You may need to do your own footwork here and point out to your attorney with specifics where and how the testimony is different.
The alleged victim didn't write the police report. so an attorney wouldn't directly ask the witness about the police reports.
Instead, the questioning would be something like, "Didn't you talk to office Jones on XXXX date? And did you tell him the truth? And you told him _________, right? But that's not what you're telling us today, right? So if you told him the truth then, how can the story you just told us be the truth?"
Officer Jones should also be on standby to impeach the witness if he or she denies saying what's on the police report.
Of course, nobody can just write a script to cross examine a witness without all of the details, including what the witness actually says at trial. That's why good cross examination skills take time to develop.
There may be tactical reasons not to cross examine a witness on a particular point. Some lawyers don't know when to shut up, and fill the holes in the prosecution's case by asking questions that they shouldn't want answered.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.