Not necessarily. There are many alternatives that incorporate both a back cover and screen cover that activates the magnetic "sleep/wake" function. And if you get an Apple SmartCover you'll probably also want to purchase a compatible back cover. That option is significantly more expensive than an "all in one" option. However, as noted on another thread, the SmartCover has one feature that other options lack: the ability to detach it easily from the iPad. You can purchase a back cover that "locks" the SmartCover to the iPad, e.g. the ZUM. But if you do so, you'll lose that "detachable" feature and gain the ability to pick up the iPad by the cover without it accidentally detaching. Your choice.
I've never had any use for a smart cover. My husband and I have three iPads between us, none with a screen cover or shield. When we take an iPad on the go, it goes in a sleeve.