Short answer: They're a complete waste of money.
Long answer: Suppose, for a moment, that such a case was perfectly efficient and prevented any RF (Radio Frequency) from being broadcast by the phone. That would prevent the phone from working. The RF is an essential part of the operation of the phone.
So if you're using one of these cases, and if the phone still works properly, then you can be sure that the case is not eliminating RF emissions.
Actually, those cases don't cover the phone's antenna, which is where the majority of the RF is emitted.
But suppose that the case could attenuate the signal generated by the phone by 50%. What would that do?
It turns out that in CDMA it would have little effect. It would increase power usage and thus decrease battery life, and it would increase the chance that the phone would drop the call. It would not decrease the amount of RF to which your head was exposed most of the time, surprisingly.
CDMA operates best when everything in the system uses the lowest transmit power it can. That not only makes all the mobiles get the best life out of their battery, but more to the point it maximizes the capacity of the system.
The cell constantly monitors the integrity of the signal it receives from the phone, and 800 times per second it sends power control bits to the phone telling the phone to raise or lower its transmit power by very small increments. The cell does this so as to make the power level received by the cell remain constant.
The signal path from the phone to the cell can vary a great deal, especially if the phone is moving while in a call. The efficiency of transmission can improve or degrade rather rapidly, and this feedback mechanism is intended to compensate for these kinds of changes. The cell makes sure that the phone uses enough power to get its signal through, but no more than that.
Since this mechanism is measuring the power received by the cell, it takes into account every source of attenuation -- including the case.
Taking again that hypothetical 50% attenuation by the case, the result would simply be that the cell would command the phone to raise its transmitter power. It would operate at twice the power level that it would need to without the case, so as to generate the same RF level at the cell as it would have without the case. This means that outside the case the phone will still broadcast the same amount of RF.
But to do so it has to operate its transmitter at a higher power level, which means that it drains the battery faster. So your "talk time" goes down considerably even though your exposure to RF remains the same.
Also, one of the ways that a call can be dropped is that the signal conditions degrade so far that the transmitter is told to boost, and can't go any higher. At that point, there will be enough errors in the data stream that the cell will give up and drop the call.
CDMA phones top out at 200 milliwatts (a very small amount of power, by the way). Once the phone reaches this ceiling, if the cell asks for more, there is no more to give, and the call will probably drop.
With this hypothetical 50% attenuation, when the phone reaches 200 milliwatts it would actually be broadcasting 100 milliwatts. This means that it's more likely to have its call drop, since it runs out of ceiling sooner.
But that discussion is academic, because the cases can't attenuate the power that much since they don't cover or affect the phone's antenna.