I played the multiplayer "Tag" mode -- essentially "keep away" -- as the "it" driver puts the pedal to the metal in San Francisco traffic and the other players try to smash into him, thus starting the cycle of tag all over again. What the shift system does is allow players who are far away from the target (or who just want a new vehicle) to disembody themselves from the car and instantly snap to an aerial view and plop into a car closer to the target. The best example I can think of to convey the intensity this causes is the highway scene in Matrix: Reloaded where the Smiths and The Twins keep taking over other drivers.
Shifting allows you to always feel like you're still a contender as the "it" person incrementally ticks up points. The strategies available also elevate what is a simple game of "keep away" into something you can see playing and laughing about with friends for hours. You don't have to be into driving games to enjoy this type of mode.