Just because a ship must slow down to use its ASW sonars, having the ability to go fast ( 30 knots) is still a useful feature.They need to slow down to use their own ship sonars. If they are going full speed you have to rely on your helicopters to use dipping sonars on positions ahead around the group. Burke use bow sonars and passive towed array sonars but lately some ships have upgraded to using active towed sonars (VDS by European lingo) whereas all of 052D, newer half of 054A and all of 056A come standard with VDS along with obligatory hull and passive towed sonars.
If the Type 052D - destroyer already has towed array sonar and variable depth sonar, then I see very little reason to give the Type 054B - frigate the ability to sustain 30 knots. It does not need to "run" with the carriers. The job of frigates, destroyers, and cruisers is to complement each other, not compete against each other.