It's difficult to use the carpool lane in Utah as a passing lane due to the fact that people aren't supposed to be moving in and out of it - there are specific spots along the freeway to enter and to exit, which does not lend itself to people using it as a passing lane.

That said - anyone who is content to go the speed of the traffic in one of the lanes to the right of it should just stay in those other lanes. The carpool lane is supposed to be a benefit to folks with multiple people in the car who are rewarded for having multiple people by allowing them to use a lane not everyone can use. With the thought that they'll be able to travel a little more quickly when the lanes to the right get congested and slow. But if the lanes to the right are going a good speed for you - stay in those lanes.