Kite474
Explorer
I see it as the players' responsibility to bring their backgrounds to the fore first, then I as DM will respond by adapting to it during play. If the player is not pursuing goals based on the character's background, then I can't be given to care about adding those elements. If the character's parents were killed by orcs, but the player never does anything in the game to get revenge or seek out which tribe was responsible or whatever, then I don't feel obligated to stick orcs in his or her path just because he or she wrote it on a sheet of paper before we were actually playing.
Oftentimes what happens is a player will create a background and then they don't do anything with it. This is part of the reason that I ask players to create backgrounds that are the length of a tweet, then reveal more detail during play. Then in my usual "Yes, and..." fashion, I will accept and add to it, framing the PCs into conflicts that speak to what has been established. I definitely won't gather four or five background write-ups and plan my campaign around that.
Wait are we still talking about Backgrounds? Or are we moving to backstories?