So the entire (original) purpose of carpool lanes is to reduce the number of cars on the road. The idea is that if three people are driving in one car instead of in three separate cars, that is two less cars on the road. In aggregate, this should speed up the commute for everyone, even people who don't use the carpool lane. Sure, you are giving up a lane, but in return you are removing 3x the number of vehicles from the remaining lanes for every car in the carpool lane.
As for your speed difference issue, at least in Southern California, the carpool lane has a double yellow separating it from the regular lanes, hopefully giving some added safety since cars shouldn't move back and forth except at designated entry and exit sections.
A lot of people think the purpose is to reduce pollution from the cars removed from the road (i.e. only one car's pollution instead of three), but in reality it should reduce pollution even MORE than that (because fewer cars on the road means everyone can drive faster, reducing the time spent with your car polluting while stuck in traffic).
However, they messed this up by adding these other ways to drive in the carpool lane. Sure, an electric vehicle pollutes less, but letting them drive in the carpool lane eliminates the benefit of fewer cars on the road; three electric vehicles still take up the same amount of space on the road and cause the same traffic as three gas guzzlers.
Then they added paying a toll, which does nothing to reduce pollution at all, and is simply a way to generate revenue. This is completely counter to the original purpose.
So if they stuck to the original idea, I think it is worthwhile. In the original form, converting the carpool lane to a regular lane would not actually reduce the congestion for everyone, because while you are adding a lane, you are also increasing the number of cars on the road (of course there are a lot of factors that go into this... how many cars would carpool anyway, and how many cars are using the carpool lane at any one time).
Here in Southern California, however, the carpool lanes are almost pointless. During high traffic times, they are just as congested as the regular lanes. In fact, they are often times slower than the regular lanes when congestion is medium... since they are only one lane and you can't switch lanes during most sections, you can end up with one slower car holding back everyone.