It's kind of annoying to me sometimes. It can get ridiculous enough that I can't take it seriously sometimes, though. If the series is good enough, I can tolerate it.
Flashbacks can sometimes be necessary to a story. That way you get a better sense of the character/how they are likely to fight/what they are fighting for. Sometimes, I suppose they can be a little overboard though.
I think they both have their own merits. If I had to choose, I would say Manga because it ties up the loose ends and really provides a full story. Anime is good too, because it adds voices and music and motion and things that Manga can not.
Oh dear lord I hope this never happens. I'm sorry, but I'm pretty sure I would hate it. Sometimes crossing 2 great things has horrible results. Like... A dolphin-elephant crossbreed. Or ice cream and tacos. :l Ew.