Flex wont keep you from flopping.
If your body pitches more than it should on an embankment it certainly does. Flex keeps you from flopping in MANY situations.
A body will pitch more while disconnected than connected.
OK, I'll give you that if we assume that the incline is perpendicular to your line, but how many times do we have that, and there's not a bank of the opposite side to catch us?
When you're going up or down a hill and you have a rock or just more ground beneath one tire, you're gonna need some flex to pull the whole vehicle down or else you'll have one tire in the air, more weight on the opposite set of tires and much less stability. Ben can vouch that he's seen the results of that.