Yeah, that's true... but since I do it for my audio itch, I usually go off with deadener and cover the entire interior of the car, doubling up in some key areas.
I do this to lessen the localization of my speakers due to their vibration. It's partly habit from past competition installs, but also since I have 8" midbasses and two 15" subs, so vibration is something I'm familiar with.
I think in the case you guys are dealing with, which is a specific component making the noise, it should be doable, and produce good results.
I have to say that in my JSW, pretty much all of the road noise now comes from my windows, which I unfortunately can't do anything about.