I've read elsewhere that the asphalt based sound deadening material that's in our cars will eventually dry out and become less effective. Our cars are all old now. I removed all of it from my car but plan to replace it with something else. Whether you choose dynamat, second skin, or anything else you're probably better off than you are with the dryed up factory stuff.
I'm pretty sure you'll hate yourself if you just remove it all. Well, unless racecar.