The CJ1/CJ2 had the thrust attenuators installed to counteract the high idle that the Williams engines put out. For the airplane to be certified the engines needed to spool up from idle to full thrust in a certain amount of time (I believe 6 seconds??) and the Williams engines couldn't do it. So they set the "idle" on the engine to a higher setting, so that it could meet the spool up criteria. This in turn however made the aircraft taxi much faster then normal. To counteract that they added on the thrust attenuators, which if your taxiing the CJ correctly negates the need to even use the brakes until your ready to come firmly to a stop.
Which makes the attenuators only good for slowing the aircraft down while taxiing, without the need of using brakes. On landing, their useless.
The CJ3's do not have the attenuators installed. It uses the new FADEC system which uses compartively lower ground idle speed, resulting in smaller residual thrust.