If you don't have an unsynchronized RNG, then any unsynchronized content can't use an RNG, and unsynchronized content is important for improving a game's experience by using local resources for things that don't really need to be shared or sent over a network.

For example, maybe in a FPS, part of the non-gameplay-critical graphics use particle generators for a cool effect that not all players see (because it's behind a building for some of them and thus doesn't even need to be rendered); if these generators used a synchronized RNG, then all players would have to do computations for every particle effect happening anywhere, just so that the combat and more game-important RNG values would be in synch when they really need to be.

There were a lot of extra checks and code involved in the synchronized RNG - the results were loggable (to track down sync failures) for instance so bloating that up with every random fire flicker from a burning castle would have been crazy.