Collider wrote...
Well let's look at the alternatives:
1. Reveal the evidence = civil unrest. Quarians not at full strength. Even without metagaming the player can suspect that telling a mass of people and the general public that someone from their government committed war crimes would bring social and political disorder, lol. People have rioted and killed against their governments for far less. Everyone knows that Tali's father was a war criminal, which would tarnish her reputation and how people perceive her.
Or not. If the quarian people were satisfied with their government, I doubt they'd try to overthrow it based on finding out about one war crime (which they could just put the blame on Rael anyway). Now, if great civil unrest already existed, then yes...that could be the straw that would break the camel's back. But if that's the case, the quarians are already a bad ally, given their internal problems.
Collider wrote...
Ideally, people wouldn't treat her differently because of what father did, but that is not the reality. The sons and daughters of disreputable people are treated differently in society, and most often it's them being treated badly. So Shepard would be associated with this daughter of a war criminal. Same deal - if you're associated with someone disreputable, you will probably be perceived differently, and probably in a negative way.
Not looking too good for Shepard and Tali here.
They may lose some respect (Tali more than Shepard), but I don't think that's as bad as potentially losing trust.
Collider wrote...
2. Don't reveal evidence = Quarians at the same strength as before. If you use charm/intimidate/rally, Tali is an exonerated hero and Shepard is perceived as a born leader. Looking pretty good pretty for Shepard in Tali.
So if you reveal the evidence, you're guaranteed to have Tali and Shepard be viewed negatively. If you don't reveal the evidence, you have a great chance for them to be viewed positively and for it to stay that way.
Is it worth the risk? If someone does find out, then Tali and Shepard look 100x worse than if they simply revealed the evidence. Like I said above, they will some respect perhaps, given Tali's relationship to her father (amongst some people, not all) but that's not nearly as bad as losing their trust.