You are correct. Your installer is pushing the Dynamat because it is a (huge) money maker for him/her. Don't get me wrong, sound deadeners like Dynamat, Boommat, Soundskins, Second Skin Audio, and all other similar materials do have a positive effect with reducing panel resonance and reduce excess vibrations by adding mass to the panel/surface they are mounted to....but Dynamat is extremely over-priced and is in most cases a high profit item; much like engine/fuel system cleaning treatments that your mechanic might recommend.
So, do it if you feel you want your installer to apply the sound deadener while he/she has the door panels removed, but be sure to ask how much they are planning to use and ask where they are going to put it. A sound deadener only needs to occupy about 25-30% of the panel it is being applied to in order to achieve the best result. The amount of total material is dependent on the thickness of the material being applied. Any higher application of the deadener is overkill and a waste because above a certain point the return on effectiveness is far less than the material being applied and you are just losing more money. Sure, it might make you feel warm and fuzzy, as more is better, but using the right amount in the right location(s) will give you the same result and save you money at the same time.
If it were me and I was applying the deaderner in my garage, I would put about an 10" square directly behind the speaker hole on the outer door skin...or an
anechoic speaker pad from
secondskinaudio.com
Next, I would try to surround the speaker hole on the inner door skin about 3" or so from the edge of the speaker and then possibly a 10-12" square piece directly in the center of the inner door skin.
Finally, I would try to follow that up with placing a similar volume of deadener material on the actual plastic door card panel, possibly close to the speaker location and then in other strategic locations to reduce the door card panel resonance.
Oh, one more place I would put sound deadener is on the roof, in a significant amount as there is none and I have a non-sunroof Touring. Rain drops sound like a hammer hitting a trash can lid.
Yes, there are lots of installer videos that show entire doors, floors, and roofs being covered, but that usually for vehicles used in competitions and IMHO is a waste.
Don't forget, there are all kinds of products out there reduce vibrations and reduce actual noise and it is really EASY to go overboard on all of that stuff. Best of luck on your install.