I don't like flashbacks as a story-telling medium. It's a bit of a cheat. It's almost saying "oh, we forgot to put this in earlier, so we'll shove it in now". It's a shortcut story telling device. In films it can be useful since it saves time, but in narrative and games fleshing out characters and telling us about people's past can be done in different ways.
If there is a flashback then it's obviously something that has all ready come to pass. If the game's story line takes place after the said event then how many other ways to show it are there except with a flashback? If it has some sort of spoiler in it or it has a serious part in the plot they can't show it at the start or it would give something away rather early. It's not a short cut if there's a reason for not letting the player in on info earlier in the game.