People have used the clear panels for years under the misguided assumption that they will make drums quieter. They are not really designed to make drums quieter; they are designed to keep other loud things out of the drum mics. Yes, the reflective properties of the panels will lessen some of the high frequencies, but that makes the drums sound unnatural, not quiet.
Now, if you put absorptive material on the panels you can change the directional characteristics of the sound across a broader spectrum of frequencies and reduce the volume to some extent, but that won't necessarily guarantee better sound or more control for the engineer or a better experience for the musicians.
There is also an unintended consequence when using panels for drums: the overhead mics pick up the reflections off the panels. This means the overhead mics must be treated more like "cymbal mics" with a tight polar pattern and HPF. It's manageable for normal FOH applications but it becomes much more apparent when doing live recordings.