Though I believe - like someone else said - your opinion of Garp Pre-Rocks' Era Flashback - will certainly play a role in how you view Garp Post-Flashback, at the very least we get three pieces of very important information: Garp's reason for staying within the Marines, a view into Garp and Dragon's relationship with Dragon was still apart of the Marines, and the full view of what Garp was doing during God Valley in which he became the Hero he is. All three of these things are extremely thought provoking and really tell us something about Garp's character.
We've honestly seen a lot of Garp's character since he went to save Koby with SWORD, and it's been excellent. After finally getting his reason for being so one-way about staying in the Marines, seeing a bit of his relationship with Dragon, and what he did during God Valley to earn his title, I think there's not much more we need to see, but Garp's fate is still up in the air. So we'll have to see what ends up happening with that!
So basically, for me, the flashback did more Good than Harm.