Most players select a background for their PC during the character creation process. In many cases it’s done for purely mechanical benefits. Being from this location may give you a resistance to fire, while this one may give you a bonus to your initiative. In all cases it also gives you access to a new skill or a bonus to one specific skill. When it comes to power gaming, no one overlooks a background benefits.
The background options that I see taken most often are the geographic backgrounds from the Forgotten Realms. Usually the player doesn’t really care that his PC is from Waterdeep, Akanul or Cormyr, they just want to additional benefit that being from those places provides. And that’s fine. It’s not the ideal reason to choose a background but it’s well within the rules. Any character can have any background.
This season during D&D Encounters: Beyond the Crystal Cave the adventure strongly encourages the DM to insist that the PCs choose one of the three backgrounds provided. This helps give the players a good in-game reason as to why such a rag-tag and mismatched adventuring party would work together. Being from one of the three areas directly impacted by the happenings in the adventure gives PCs a strong motive for accepting the mission.
While at first a few of the players in my group resented being told they had to choose one of only three options (none of which they felt provided particularly appealing benefits) after only two sessions these same players really started working their background into the way the characters behaved. Without any prompting from me, the players instilled within their characters a sense of national pride.