You are right. And I want to fill this gap..
scenario: main comp motion blur is off. precomped comp motion blur is on. in main comp there is still motion blur visible, but clicking on that switch to turn on, and again click to turn off, there you go..motion blur is off of all comps.
Sorry, but there isn't an easy way to do what you seek. You'll have to do this within the nested comps.
Incidentally, this is a feature, not a bug: there are many occasions when you want motion blur only on designated layers. Adobe figures you know what you want when you enable motion blur in a nested comp.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA