Good point, I am with you on that. I have installed three of those in the last 10 years. One in the former house. Two in the new(40 year old) house.
According to the local insulation company the other important thing to do is air seal the attic. That means go up in the attic and caulk the seams where the partition stud walls meet the ceiling drywall to make sure there is no air infiltration from the heated second/first floor into the attic. Around ceiling light fixtures, ceiling fans and bath vent fans it may be necessary to foam around the openings between the drywall and the fixture. By doing this alone he stated it added about R10 to the attic insulation.
I did this last fall to the house we bought the previous spring. I used about 14 tubes of caulk @ $2.98/tube.
The reason is that most houses prior to 1990 they did not poly the ceiling of the house with a 6 mil vapor barrier. When I bought my 1972 vintage house last spring there was 5 1/2” of fiberglass insulation in the attic. The previous owners had lived there for 25 years. I blew in about 12” of bonded cellulose on top of that. I will probably add to that this spring or fall.
Hmmm...I never even thought of that. Where were you before I added the additional insulation? Ha-ha.