Until now, multitasking in iOS has been fairly limited. Now, it's going to work for all apps—and Apple says it's going to provide background updates to all apps—and Apple says all those background cycles aren't going to hurt your battery life.
Apple's multitasking is smart—it learns from your habits and updates the apps you use frequently, and not so much the ones you don't. The updates also adapt to the network conditions. So it's not going to try to update in the background when you've got crappy connectivity.