Fewer than half of the NFL's defenses operate under the 3-4 base system, but the Steelers have been notorious for carving championship teams out of this style since first implementing it in the early 1980s.
It might not be the most common formation used nowadays, even by the Steelers' standard. A nickel-based defense oftentimes has come to use in order to protect against the pass, with a nose tackle removed in order to place an extra cornerback in the secondary.
That doesn't mean the nose tackle position is becoming -- or will be -- of less importance.