You're obviously right in that the Passion is not an "overview". It's about as focused as you can get.
What I do think is important even from a secular perspective is to understand exactly what kind of suffering Jesus knowingly endured. Most modern folk have no real conception of the horrible things that can done to the human body, and that were done throughout history. But people back then knew because they saw it, which is one of the things that makes Jesus' sacrifice so powerful to them. I mean, enduring that abuse and forgiving the people who did is in incredibly powerful.
Actually seeing what Jesus endured voluntarily in a far more graphic manner than the sanitized version that traditionally has been portrayed gives the viewer a better perspective on why Jesus' sacrifice had such an impact. And I think that's true regardless of whether or not one approaches the movie from a religious perspective.
I suppose the analogy may be how Saving Private Ryan and the more "modern" wave of war movies helped non-veterans better understand exactly what "war" is, and the scope of the sacrifices made by the guys who fought.
I'd tend to agree with your analysis, it's not a complete picture, and IMO likely difficult to understand without some background. However rightly or wrongly the expectations for the film as an evangelical tool are very high.