| If there is sound reflecting off the glass and the dash it can be very... edgy. you cant stop the glass from being reflective but you can limit the dash's affect. The sound the listener wants to hear is the sound recorded on the CD. Anything that helps to take the cars affect (effect?) out of that equation is a step in the right direction.
I had drivers in the dash and the dash mat was essential. BUT several of the dash mats I made absorbed too much and made a muted HF sound... so what I needed was refraction (not so much absorption or reflection)
My current car doesn't need a mat, if you use kick panels for high and mids you probably dont need a dash mat, but you might need one under the dash.
Just throw a towel or two on the dash and listen critically, is there a difference? is it better or is it worse? | 
