Flashbacks: useful and informative or a weakness?

I have read though most of the books by now, and really, I thought 'The Other Side of Silence', while slow, was somehow stronger than many of the flashback-heavy books. There were only two that I can remember, quite short, and I felt that the book could have done without them. Books like 'Field Grey,' which was almost entirely flashback, told a story, but you lost the plot in places: why exactly are we talking about this man again? Oh, that was three flashbacks ago. I forgot. Has anything happened in the present timeline since? No? Okay, back to the forced exposition...

I enjoy the books most when Gunther is settled in one time and place for more than 50% of the book. Does anyone have any thoughts?