I've seen an epileptic fit, it's not good squat form
but all kidding aside, I see your point, and I agree that moving the weight in a straight line does not mean good form is being used, this misconception is probably what gave birth to the smith machine.
I can use the bench press as an example. If you want to lift the most weight, you want to tuck your elbows in a little and touch the weight low, like on the abs, where your belly is the biggest when you're pushing it out. If you were to lift in a straight line from there, the bar would not end up right above your shoulders (with the arms perpendulicar to the floor, in a good position to support the weight at lockout) but further down (towards your feet) holding the bar here would be impossible! catch my drift?
is that the kind of thing you mean?