Yeah, I'm not worried about using a knee. It's having Dad and Firefighter Bob jumping up and down on the seat that really makes me worry.
I mainly get frustrated with people who won't accept anything less than rock-the-car-solid, even to the point of getting rid of seats that have acceptable but not rock-solid installs. (Their right, of course, I just don't get it.)
Does anyone remember the study published in Safe Ride News or SafetyBeltSafe a year or two ago about how seats with extra slack (more than an inch) performed the same as those installed properly? I don't remember the amount of slack, and the idea certainly isn't to disregard tightening the seat properly.
It's a good reminder, though, that at a certain point it's like splitting hairs.