The fact that the community are able to vote on what vehicles are included at each rotation is amazing. It gets people involved within the community and playing the game more. What's more, it gives the players a bit more variety regarding drivable vehicles in multiplayer, spicing up their experience. It is also good fun with your friends.
Alternating Vehicles also gives the multiplayer world a slight "single player" feel with what would usually be AI controlled vehicles on the road, but being driven by players instead.
All things considered, I think the Alternating Vehicles wa